Noam Chomsky and others would say that the difference between "resistance" and "terrorism" is that:

Resistance fighters are fighting for causes or goal(s) that are supported by your government (ie. of the country you live in).

Terrorists are fighting for causes that your government opposes.

This is based on Chomsky's analysis of the United States media. For example, the Contras were "resistance fighters" even though they were not supported by most of the Nicaraguan population, and specifically targeted non-combatants and civilian infrastructure. But they were on "our" side, so therefore must be "freedom fighters". The (your country) government is by definition "democratic" and "good" and so are their international allies, even if they are evil bastards.