Originating from the Warhammer 40,000 Space Marine race/faction, the term has became popular for any heavily armoured and armed soldier in a futuristic (near or far) setting.
The Term is normally used for the elite as described before, but can be used to describe any trooper in power armour, for example the Starcraft marines are often nicknamed (or accused of being) Space Marines and they're not elite at all (mainly being physchologically adjusted inmates). Similar story can be said of C&C3 Zone Troopers, Mobile Infantry in Starship Troopers etc, although the term is very rarely used for Fallout Brotherhood of Steel paladins as they have nothing to do with space (but their attitude and appearance almost mirrors the Space Marines word for word)
The Terms is often only used for humans.
RTS Player: Hmm the noob seem to be rushing my base with cheap stuff... time to drop-pod some space marines then...