Terrorism is an act according to which someone makes use of fear and uses fear tactics so as to control the minds of people, and to persuade them to act in any specific way. On the other hand, a freedom fighter is the one who fights on behalf of some civilians and is in direct opposition to the government, military, or any ruling party that is governing the land. This is the prime difference between a terrorist and a freedom fighter.
However, the definition of terrorism is subjective and is always defined by the one who suffers the casualties on hands of the other party.