The term "human werewolf syndrome" refers to the condition wherein a werewolf, while in wolf form, maintains its human consciousness.
In contrast, "werewolf syndrome" is an little used term which refers to the condition of being a werewolf. The terms "lycanthropy" or "werewolfism" are more common terms to describe this same condition.
Now that we've got our terminology cleared up, a little explanation is in order.
Human Werewolf Syndrome is an extremely rare condition. It is a genetic abnormality caused by a mutation in the werewolf gene. This mutation occurs rarely and randomly in certain cases where two werewolves copulate to give birth to a werewolf child.
Werewolves with this affliction are easy to spot. The mutation of the gene causes a distortion of the coat color of the affected werewolf. This distortion causes the fur to be all black or all white, rather than the usual mix of browns, grays, and blacks of the common werewolf. These "mutant" werewolves are called "black werewolves" and "white werewolves" respectively.
Genetic mutations occur randomly in all species. This is actually for the benefit of the species' survival, as some mutations create a superior animal. One could argue that this is the case for the black and white werewolf, as their mutation causes them to be born without the synaptic receptors in the brain that absorb lupocorticosterone, a chemical believed to cause the violent rage of their werewolf counterparts.
Thus, black and white werewolves are able to maintain the same ability to think consciously and reasonably as a regular human being, no matter what form they take physically. The two variants of the species do differ significantly from one another though in that white werewolves never take human form, even from birth. Black werewolves still physically change at night just as a "normal" werewolf would, but they do not lose their ability to think with their human mind.
Full explanations of each are available on the black werewolf and white werewolf pages.