Grunt. Your basic military-type goon, bald. His distinctive feature is a red S on the uniform. Different grunts may carry different weapons.
Guards - They appear in mission 6 and carry through to the rest of the game. They look like thugs except they wear silverish armor. They carry submachine guns and shotguns for the most part, though later on you'll find them with rocket launchers and chainguns. They're not very dangerous, and only about as hardy as earlier enemies.