CS, or Creep Score, is a term usually confined to the MOBA scene, which itself stands for Multiplayer Online Battle Arena. MOBA has become a dominant genre in video games, encompassing games like Dota, LoL, HoN, and HoTS. In MOBAs, every couple of seconds, fodder in the form of weak monsters gets released for players to feast on and gain experience from. No matter if those monsters are called creeps or baddies or minions, everyone knows that the amount of them you kill equates to your creep score.
Look back to the old days of the MOBA, when the genre (Defense of the Ancients, specifically) was just a budding mod on the servers of Warcraft 3. In DotA, the NPC cannon fodder was called a creep; therefore, your creep score was how many of them you killed. The minds behind that singular mod have gone off to create most (if not all) of the successful modern-day MOBAs, and many of their fans have followed. The terminology of calling something a creep has stuck, reminding MOBA players of their community-made roots.