Albinism is due to one of several gene mutations that affect the production of normal pigmentation. True albino, or amelanistic, animals lack melanin and are white with no markings and with unpigmented pink eyes. In some species there is also a form known as blue-eyed (or "partial") albinism. There are also various degrees of patchy albinism - piebaldism - that occurs due to localised albino mutations in skin cells during embryo development. If the mutation occurs early in development, the patches are larger. If it occurs later, the patches are smaller.
It appears that it is now known that white tigers are not albinos, and nor are they a separate subspecies, but are the result of a recessive gene. They lack much of their normal colour so can be considered albinistic, but the presence of pigmentation causing the stripes and colour in the lips, paw pads and nose, means they are not albinos.