Michael Jericho at A Western Heart makes a very good distinction:
Rebels or Foreign Terrorists?
In order to be the former, the one doing the killing needs to be a local, and needs to be acting in a vaguely military fashion (ie: limiting attacks to enemy military personnel).
In order to be the latter, it is helpful if you aren't a native of the country you are fighting to “liberate”, and there has to be a good indication that you are deliberately targetting non-combatants for torture and death.
He then quotes from a Yahoo/AP story regarding “Rebel” activities uncovered by coalition action in Fallujah and concludes with this trenchant comment:
Posted by DaveH at November 15, 2004 04:30 PMNot rebels, and not resistance fighters. These people are invaders with a different agenda at best, jihadist terrorist scum at worst.