Okay, you could just Bing it, but here's the gist as told by me!
In the World of Darkness, werewolves are very real. They keep their existence a secret from the human herd, the better to move unchallenged in the shadows. They are creatures that walk between two worlds - flesh and spirit, wolf and human balanced in their forms. Their purpose: to walk between the worlds of flesh and spirit, policing the wall between worlds. Alien spirits hunger for sacrifices, and lie in wait to possess unwary mortals. Foolish humans, and worse, seek to enter the spirit-world to steal its power. Monstrous abominations lurk in the forgotten corners of the world, and things from Below hunger to infect and ruin all of creation. Some werewolves even choose to hunt you, swearing eternal vengeance for a terrible crime committed by your forebears, and there is no mercy in their furious hearts.
But you are not alone. Your pack, closer than family, stands at your side. Bargains with the spirits may be struck, influence and power among humans may be gained, mystic Gifts and arcane rites may be mastered. You and yours are the finest hunters in the entire world. And deep in your heart, you know that the hunt is more than just a desire - it is a NEED. You are blessed by Moon, the god/dess whose face fills you with furious joy. In you sings the blood of Wolf, the greatest hunter that ever was, or ever will be. Claim your territory, clash with rivals, seek knowledge, gain influence, but most important of all, HUNT. Your enemies abound, Uratha. Make them think twice about who is predator, and who is prey.