An activist is a person who campaigns for some kind of social change. When you participate in a march protesting the closing of a neighborhood library, you're an activist.
Someone who's actively involved in a protest or a political or social cause can be called an activist. Demonstrations, strikes, and sit-ins are all ways that an activist might work toward the change she believes in. The root word of activist is the Latin actus, "a doing, a driving force, or an impulse." Someone who acts on what she believes is an activist.