You need to build and upgrade different kinds of towers to defend your lives agains incoming creeps. If you don't and a creep passes through your board completely, you lose a life. On the other hand you have to send different kinds of creeps to your opponents, so that they will lose their lifes.
it's back!
Looks like the game is down temporarily and they've swapped it out for a YT vid... Checing their Facebook Wall, that seems to be the case http://www.facebook.com/CreepTD?sk=wall - try again in a few hours