CTF is a Capture the Flag mini-game server. The concept of CTF has been around since 1998 when the Original Classic source code was leaked, but this particular version in the images was created on Unknown's Registry.
You start out in the lobby, and depending on where you go, you can buy different items to use in game.
To purchse an item, just stand on the tile in front of the specific item and type "buy"
You enter the red or blue tiles in the lobby to join a team.