The definitions below are taken from the Merriam-Webster OnLine dictionary.
rebel (noun): one who rebels
rebel (intransitive verb):
1 a : to oppose or disobey one in authority or control b : to renounce and resist by force the authority of one's government
2 a : to act in or show opposition or disobedienceb : to feel or exhibit anger or revulsion
insurgent (noun):Note that "insurgents" do not necessarily spread violence and terror, and that this word should probably be reserved for those who are "not recognized as a belligerent."
1 : a person who revolts against civil authority or an established government; especially : a rebel not recognized as a belligerent
2 : one who acts contrary to the policies and decisions of one's own political party
terrorist (adjective or noun): [defined via the definition of "terrorism"]Sound familiar?
terrorism (noun): the systematic use of terror especially as a means of coercion
When characterizing the situation in Iraq, journalists seeking legitimacy might do well to take these definitions into account. (Sorry, Dan, it's already too late for you.)