Contract killer Agent 47 uses a mixture of stealth, weapons, special powers, and creative use of the environment to perform assassinations across a variety of locations.
The Hitman series, as with many video games, is about killing people. Where Hitman is different, is it lets you kill those people in a variety of different ways. Sure, you can walk up to someone and shoot them in the face, but that's rarely the most interesting (or fun) way to do it. Much more interesting (and fun) is to drop a chandelier on that person, booby-trap their toilet, or poison one the bottles of water they always seem to have; with no-one having seen anything, and no-one suspecting a thing.