The most common distinction is that a werewolf is a human-like wolf and a wolf demi-human is a human with animal parts.
Wolfkin generally look like humans, but have wolf ears, tail, in some cases also feet (paws?) and leg structure (digitgrade), fur in some places, for example on forearms, claws, occasionally different skull shape. They also often have wild hair, plus instincts and senses of a wolf. They should be vieved as a race just like humans as they form societies.
Werewolves meanwhile are more like a wolf walking on two legs, usually digitigrade with long arms and claws, completely covered with fur, even including face, which resembles that of a wolf, although usually has adjusted structure to let the werewolf walk upright. Most of the times there is hardly anything that would hint there's a human under the skin. They can traditionally transform between human and werewolf form though, often spontaneously during full moon. They don't form societies, but usually integrate into existing ones to prey on humans.
Edit: In some cases wolfkin are furries, although that's not common. In such case the main difference between them and werewolves is body shape, which is humanlike but covered in fur, but there are hardly any differences in behavior comparing to their more humane versions.
It can generally be viewed as a sort of spectrum across various settings though; the closer to wolves, the more likely it is they will be called werewolves, although there are some stories (Bloody Aether, for one) where there is a race that looks literally like two-legged wolves, but is perfectly capable of reason and forms societies.