A freedom fighter is a person who thinks their native ethnic group is not free, and is working to have freedom for their group. Most often this means that a freedom fighter wants their people to have own nation and independence and/or get rid of oppressors. People who think like this are said to think in a nationalist way
Freedom fighter is a relativistic term - this means a person decides by his or her point of view if he call some persons or groups freedom fighters or not. Freedom fighter is a positive term - this means that he or she calls a person "freedom fighter" only if he or she supports the goals of the freedom fighter. If people do not support his goals, they probably use more negative terms like insurgent, terrorist, rebel or criminal.
Freedom fighters are often called terrorists by their opponents if they hurt civilians. However, not all freedom fighters hurt civilians. Some freedom fighters damage the opposing military. Some freedom fighters damage property - this is called sabotage. There are also freedom fighters who do not use violence at all. For example, Mahatma Gandhi and Dalai Lama.
Groups
These are examples of groups who call themselves freedom fighters.