Garou are, in the most basic expression, werewolves. They fight for Gaia against the evil known only as the Wyrm and the madness of the Weaver. Each Garou has a breed, a pack called a "tribe", and an auspice.
From the Werewolf: The Apocalypse Wikia:
"The three main spiritual forces in the World of Darkness are the Wyrm, Weaver and Wyld. The Wyld is raw unbridled creation. The Weaver spins reality into patterns. The Wyrm is supposed to destroy the excess order or the excess chaos. However, the Weaver went insane and tried to spin all of reality into its web, including the Wyrm. This drove the Wyrm mad so that it now seeks not to balance, but to destroy everything. The Wyrm has many agents and many faces, all bent on the utter destruction of Gaia, the Earth. The Garou are fighting to keep the Apocalypse from coming, but they are dying out and losing the war. Some tribes believe that the coming Apocalypse will be the end of everything, and fight desperately for even the smallest victory. Other tribes believe the side that wins at the final battle will be able to reshape the world in its own ideal. Some Garou blame the Wyrm for all evils and some blame the Weaver for spinning the world into stasis. Thus the Garou can agree on nothing and will not provide a unified front in the coming End Times." 
Children of Gaia
Get of Fenris