Squad based tactical combat. Most of the "tactics" amount to setting a unit behind cover or in a building. Until you get terminator honors for all of them, and you can just have them stand in the open (they can't use cover in terminator armor anyways) slaughtering anything and everything without any problems. Oh, and you have the same four squads (five if you count the commander, six if you count the canned corpse) throughout the game, each led by an annoying asshole, whom you level up and give equipment to. Your troops appear to be fluff marines, and they easily slaughter the hordes of shitty enemy units (which are the units you play with in multiplayer, by the way).
It's fun for a while, then it gets repetitive and methodical, with the same strategy tending to work against anything (especially once you get the canned corpse to draw fire and generally bash the fuck out of anything in its way). Oh, and none of the characters can die, only the redshirts surrounding them. Named characters just get knocked out and lie on the ground like pussies until another named character wanders over and kicks them in the head a few times. Boss fights are a tedious grind, as they need to be shot thousands of times before they finally die, I remember one of the last ones took over an hour to kill (an unusually obnoxious fight, that one).