Mutant tigers. Not Siberian tigers.
White tigers are Bengal tigers that have two recessive genes for partial albinism. They are white with brown, grey, or black stripes. They are blue-eyed, and are often cross-eyed. White tigers also tend to be larger than other tigers, which may give them some advantage in the wild.
While white tigers to occasionally appear in the wild, most white tigers alive today are descended from one individual named Mohan, who was owned by the last Maharajah of Rewa. Mohan was bred with his own daughter in order to produce the first generation of captive white tigers, starting a long tradition of selective inbreeding.
Not an endangered species, despite being rather rare (especially in the wild). They are a sub-sub-species, or breed, of the common orange tiger (Panthera tigris), selectively bred because they are worth $60,000 each.