Corewar is a game played between two or more computer programs on behalf of the players who create them. Known as warriors, these programs are written in Redcode, a low-level language similar to assembly. Warriors battle to eliminate all opponents in the core memory of the MARS virtual computer.
The hills provide the typical battleground for Redcode programs. When a warrior is submitted to a hill, it fights against warriors currently on the hill. It scores according to the number of wins, ties and losses. The challenger will enter the hill if its score is higher than the lowest achieved by a current hill warrior. The lowest scoring warrior will then be pushed off.
A variety of different hills adds to the diversity of gameplay. Some are round robin, where each warrior fights every other in a one-on-one battle. Others are mêlée (multi-warrior) where all warriors battle simultaneously in core. To this is added a selection of core sizes, number of processes available, etc.
The highest scoring warrior on the hill claims the title King of the Hill. A warrior's age is measured by the number of successful challenges it survives. Interestingly, quite a few of the longest surviving warriors were never KotH.
Bloggers: there's a brief introduction to Corewar available which you're welcome to post on your blog.