A militant is someone who is not afraid to use verbal or physical violence to further a cause. Militants have been closely associated with extremism and sometimes even terrorism historically, due to the fact that they are closely linked with violent activities. A number of organizations and people self-identify as militant, with the goal of projecting a very characteristic image of themselves.
Just like “military,” the word “militant” comes from a Latin word which means “to serve as a soldier,” but militants are not directly associated with a particular military, and they often work alone or in small groups. Many view themselves as martyrs for a cause, willing to go to great lengths to achieve a desired goal, whether it be the liberation of a nation or the promotion of veganism. One's stance on militants tends to vary, depending on who the militants are and what cause they are supporting.
Although you may link the concept of militancy with armed ruffians agitating for social and political change, you may also have heard it used in rhetoric simply to refer to someone with very strong and clearly stated views. These militants may or may not actively engage in physical violence, but they are certainly very aggressive verbally, sometimes alienating other people in their movement, and many do justify acts of violence with rhetoric from their causes, even if they do not personally engage in violence. Militant activists are often a source of frustration for people who agree with their cause but do not support extremism, because radical activists can give an entire movement a bad name.
In the sense of someone who does engage in violence, a militant typically has at least some training with weapons and in military tactics, although the degree of skills varies. Militants are often led by charismatic leaders who demand absolute loyalty and cohesiveness in their groups, and they can be found in a wide variety of locations. In some cases, militants are actively supported as they attempt to overthrow governments or enact regime change, because other governments believe that their cause is worthy, while in other instances, militants are treated as terrorists who undermine safety and security.
Some militants have become famous as icons for their causes, and sometimes as martyrs as well. For people who do not come from cultures with a long-established militant tradition, the idea of becoming a militant may seem a bit odd, but for people with a history of cultural oppression and frustration, many militants are glorified, because they are perceived as heroes.