I prefer to classify shooters by gameplay elements rather than settings.
RPS - action-RPGs that focus on shooting
EX: Borderlands, Mass Effect 2
Tactical Shooter - lots of tactical shooting I guess. I'm not very familiar with this one.
EX: ARMA and SOCOM
Plaform shooter- shooting and platforming
EX: Mirror's edge, Infamous, Ratchet and Clank
Stealth shooter - shootin' and sneakin'
EX: MGS4, SC: Conviction
Action shooter - pretty much every shooter that doesn't fit into the above sub-genres.
RPS - action-RPGs that focus on shooting
EX: Borderlands, Mass Effect 2
Tactical Shooter - lots of tactical shooting I guess. I'm not very familiar with this one.
EX: ARMA and SOCOM
Plaform shooter- shooting and platforming
EX: Mirror's edge, Infamous, Ratchet and Clank
Stealth shooter - shootin' and sneakin'
EX: MGS4, SC: Conviction
Action shooter - pretty much every shooter that doesn't fit into the above sub-genres.