It's a way to play 40k online on a 2d, virtual tabletop. It comes with sprites for the models, but does not include the rules/army lists.

http://www.vassal40k.com/

Sadly, however, the maker of vassal 40k has received a cease and desist order, so he will not be able to update it anymore. The servers will still be up, though, so download the module while you can so you can play.