It is the year 2084. You control a secret organization charged with defending Earth from a brutal alien enemy. Build up your bases, prepare your soldiers, and lead them to victory in turn-based tactical combat.
An excellent recreation of the X-Com and UFO games, with pretty good multiplayer added. It still lacks a few features such weapons platforms and destructible buildings, and the endgame is IMHO incomplete. Other than that it is IMHO the best of the various X-Com recreations out there and best of all it is free.
One major caveat. 2.4 is heavily outdated. I strongly recommend downloading a build of the 2.5 development branch from the project website, ufoai.org , as it contains many updates improvements and bugfixes and is normally quite stable.