Skinwalkers are Shapeshifters that can assume the shape of other humans. They can also convincingly copy vampires, sans fangs. According to the legend of the Navajo tribe, Skinwalkers are evil witches whose powers allow them to transform into any animal on earth. This includes other people.
A shapeshifter must kill another shapeshifter from their own family in order to become a skinwalker. Luna's mother died in childbirth when she was born. This gave Luna her skinwalking powers.
Tommy Mickens became a Skinwalker by killing both of his parents, one of whom was a shapeshifter. He later unexpectedly/unintentionally shifted into Sam Merlotte and thereby caused some trouble for Sam. ("I Wish I Was the Moon")
If the skinwalker stays shifted as a human for too long, they will experience several unpleasant side effects. These include loss of conciousness, high fever (even for a shifter) and vomiting blood. Extended skinwalking wreaks havoc on the skinwalker's body and can be fatal.