Here are some types of activism. There are more methods, examples, etc., but most forms, acts of and energies direct toward activism for social, environmental and policy changes fall under these categories below:
Volunteering: if you pick your place/org to offer your volunteer services at carefully, believe in the "cause" regarding the institution/place you're volunteering for, this is an active form of activism. Whether you make larger statements and do more emphasized actions while in your volunteer role is up to you. You can be a quiet volunteer or an outspoken one (drawing attention to your volunteering, the mission statements of the place where you volunteer, the issues surrounding the place, etc). It's all up to you.
Grassroots: these are student, community, neighbourhood groups where some organization is required and where team leaders emerge to "table" issues (put topics out on the table for discussion, problem solving, planning actions to resolve problems, following through to enacting plans, etc). Often, grassroots groups provide literature dealing with the issues the group will take on, discuss, attempt to change, etc. Grassroots groups can be very small or very large. The structure of the groups (not the size) and its members (community, individuals in the community) are the main indicators of whether or not an activist group is grassroots or not. Grassroots groups usually adhere to consensus based decisions and allow any member to speak out, ask questions, suggest solutions and the answers/solutions to problems might come from any individual in the group. Grassroots groups are often effective because the members are experiencing effects of whatever problem is on the table and also understand the limits, strengths and cultural/social details of the individuals/population that is experiencing a problem. Where large activist bodies may suggest and implement solutions based on other models of things that work elsewhere, in a different demographic population, Grassroots groups are likely to know, better, what will work in their population.
Letters/Petitions: can be performed by anyone, members or non-members of any group, by individuals in the general population, by politician, moms, kids, anyone. There are good/effective and bad/ineffective ways to do letters and petitions. Letters can be sent to the managers of companies where you have concerns about the company policies, products and services and wish to see a change in those policies, products and services. Letters to the editor of newspapers and magazines display your viewpoints to a large audience. Petitions require more planning and individuals before they are presented to institutions, companies, corporations, authorities and various entities where you'd like the information on the petition to spark a change in policy, products, services, practices, etc.
Direct Lobbying: going in person to political offices, government buildings, requesting to see/meet with a politician, going into any political meeting of politicians where the general public members are allowed to sit in on the meeting. Going to meet those in control of large corporations in the same manner.
Litigation: usually used when other methods of activism fail - direct form of activism. Attempting to use the law/lawyers and representatives of the law to help you show where another legal problem exists in government, community, a corporation's products, actions or services, etc. In short - attempting to make an entity responsible for his/her/its actions as punishable by law. Sometimes used against large groups of companies such as advertisers ie: litigation is successful, on occasion, to create rules/laws saying that all advertisers must adhere to a set of principles/laws or that advertisers must stop doing a certain type of advertising or stop advertising to a particular demographic group, etc (perhaps advertisers of certain adult products can no longer place their ads in comic books that will likely be read by a mostly underaged/minor group of people).
Consumer Boycotts: organizing a boycott of products and services from a particular company or manufacturer - stop buying items from a store/company whose practices and activities are seen to be unethical. Stop buying products that encourage something you don't like - no matter which store is selling the products. ie: refusing to buy Barbie toys out of the belief that the scantily clothed, hour-glass shaped anatomy and such promotes unhealthy images of females to a population of impressionable children. Consumer Boycotts often become large EVENTS requiring a lot of organization. For example, boycott participants of the Barbie toy might refuse to feed money into a number of institutions - while making this a public statement (a demonstration), so the parties who are affected might be the company building of the creators of Barbie, the largest store chains that sell Barbie (maybe Toys R Us, Wal-Mart, other large chains), websites online that sell the Barbie toy and accessories, even eBay sellers who are selling second-hand Barbie toys. Usually, all of this "refusal to buy" is accompanied by other activist tactics - letter writing, demonstrations, civil disobedience, others - alone or in tandem as multiple activism tactics all at once. Boycotts can be incredibly powerful activism tools, events, activities.
Selective Purchasing Ordinances: in some cases this is like a boycott on a larger scale. This is asking/organizing for a large entity to support the idea of not doing business with/not purchasing from another large business entity whose practices, products, services, presence in the community, actions - are considered unethical or unsound. This could be like asking the largest University in your area to stop buying books from a large chain store that is doing something harmful to the environment while manufacturing, transporting or dispersing books. Often, as with consumer boycotting, Selective Purchasing Ordinances often gain wide-spread public attention. The stop to flow of money from the University to the book store will effectively cause the book store financial hardship and the book store will have to review its practices if the Selective Purchasing Ordinance is maintained or else continue to experience lack of funds coming in from the University. Some targets of this type of activism are big enough to withstand financial hardship but when this type of activism grows large and extreme, the parties involved AGAINST the target entity usually have incredibly powerful influences on the general public, other large companies and business entities in the community - and other people might join suit because the University withdrew business from the target entity. In all of this, it will be a rule that people employed by the University will never act on behalf of the University to engage in business with the target book store who is black listed by the University.
Ethical Investing: is related to selective purchasing ordinances. Ethical Investing by large bodies/business entities can be made into legitimate political/activist statements. A business entity may make it known to the public that it will refuse to engage in business with any other business entity or individual who does not uphold certain ethics and that this is as a matter of regular day to day business, not in reaction to a sudden issue in the news, in the business/science community. If, say, some other companies use child labourers and your Ethical Investing company is against this form of child and financial abuse, the company will likely never knowingly engage with another company that uses child labourers, because it has already created a structure of practices that lean toward ethical investing. Likely, the rules on types of people/business entities an ethical investing guided company will or won't do business with are written into employee contracts, in mission statements, and are available to/visible to the general public all or most of the time. Those who engage in Ethical Investing have likely attempted to prevent situations of "selective purchasing ordinances" and public boycott demonstration activism by thinking ahead and being activist-minded while in the process of company development, company regulations and practices development.
Economic Sanctions: these are much like Ethical Investing details above but at a standardized, legal and governmental level. Basically, this is where the activists manage to get the government to make it LAW to do or not do something that has been seen as a problem for/in society. Or - to get the government to attempt to do something in another region for a human rights or economic issue that appears plainly unethical. These are BIG DEAL items - requiring a lot of organization and activism know-how (and usually political environment expertise) to enact and support. These are mainly "country-wide" and "nation-wide" acts of activism... not for the foolhardy, inexperienced crowd or for beginners. Also... I will admit, I am too NEWB to expand upon this topic. It gets complicated, involves a lot of prior actions and levels of activism, and requires people with the time, energy and money to accomplish contacting, persuading, engaging the government or large corporation to act on an issue. These are basically like Ethical Investing, Selective Purchasing Ordinances on a large scale but also include "ethical behavior and action ordinances" in concept. Big stuff.
Demonstration: this is where people come out into the public eye to protest against something. This is too large a topic to address here but I'll mention at least a few types of demonstrative activism here. Sit-ins, marches, rally events, street theatre, teach-ins/seminars, strikes, hunger strikes, etc. These are what people think of mostly when they hear the word "activist" or hear people talking about activism. Sometimes, with demonstrations as the only thing that comes to mind concerning the topic of activism, people think they aren't suited to be activists. Some don't WANT TO do a sit-in, go to a rally, participate in a strike - and this is okay. There are other forms of activism available - as this general list shows!
Civil Disobedience: is a HUGE topic that won't be covered in full here and will just be mentioned as a form of activism. Civil disobedience is often a form of public demonstration but it can occur in small or large scale. It can also be well performed or very badly done, causing harm to the protester and even to bystanders. Civil disobedience with a group of protesters is usually best done through step by step planning. On occasion, a single protester (such as Rosa Parks, the black woman who refused to give up her seat to a white passenger when ordered by the bus driver to do so in 1955 - in Montgomery, Alabama, U.S.A.) can cause extreme public notice of an injustice or event in society and lead to changes for the better. Civil Disobedience is related to Civil Non-Violent Disobedience but often, what people attempt as Civil Disobedience of the Non-Violent variety turns into something violent. Those wishing to act upon Non-Violent methods usually have to be carefully trained or carefully prepped for an event or instance of this type of activism. Personally, I do public speaking and tabled talk events regularly on Civil Non-Violent Disobedience and believe it is one of the most powerful, empowering types of protest. It can, however, be done in the right place at the wrong time and still turn out as a harm to people, an issue, etc. Usually, pop-culture treats Civil Disobedience incidents and events in a poor way, choosing to associate all Civil Disobedience (including organized and well planned non-violent type) with images and stories of RIOTING, rioting combined with looting, and events in our past where Civil Disobedience instances did, indeed, turn out as harmful events. This is NOT actually how all protesting and activism of this type plays out. Certainly, it is not how responsible Civil Disobedience supporters envision any given protest event.
Agitate/Agitation: sometimes we go into another cultural environment or just any other environment where there might be oppression that we can see/notice but where the general inhabitants of that culture/environment are unable to notice or act against the oppression. In these instances, we can "agitate." Talk to the people about human rights, be supports for them. We have to be very careful to also LISTEN to what these people say about their immediate environment so that we don't suggest something or some action that is going to be harmful. Most "change" occurs with a price in time, mental and emotional strain, discomfort of some sort. We need to be really careful with agitation - so that we don't cause undue harm. It is best if we can remain available to support people who do decide to protest in their environment - for the time of discomfort that is part of almost every process of change in any environment.
Career: Some people make a career out of activism. This can be a situation where an activist-minded person actually goes several stages further and creates an organization dedicated to activism or to a certain cause. Sometimes this will mean a person goes forward to become a politician with a definite title. Other times it means joining with or getting a job at a particular institution or company that is working toward defeating a problem in society/has a definite cause.
Arm-chair Variety: There is an arm-chair variety of activist. These individuals, I rarely encourage but, at the same time, I do try to listen to what they are saying. Often they are people with bark but no bite who have strong opinions on issued but who never get out of their chair to take action on the things they talk about. Many people really hate arm-chair variety activists but I don't take the same view. I think arm-chair variety activists have a lot of good ideas but may be lacking in knowledge of implementation of their ideas. Sure, some people just bitch about things from the safety of their seat at home, but I think that most people, if given information about how to actually get out of their seat and do something in the way of how they're talking and thinking... will get up and do something.