Well, as far as I can tell, it just adds another player to the game who can do more or less what you do: i.e. run around, purify tiles, order survivors around, and so forth. So it's like two guys sharing the same world. I'm not sure how player stats are affected; I didn't notice any difference in the game's difficulty. Just be careful; if the other guy is a noob, then you might find your morale draining away.