Shake Hands are undead remnants of woodland skeletons that awaken upon passersby in the path it shoulders. They remain in the spot (always right on the side of a path in the woods) the original creature died in, and are confined to it, though those they encounter don't necessarily know this. Their distinguishing characteristic seems to be a trademark skeletal hand of whatever creature it was, outstretched, jutting into the path as if to shake yours. The outstretched hand is no doubt an invitation of some kind.
The nature of this invitation, Sorcerers believe, depends on the spirit inhabiting the bones of the original animal, person or monster. Tales of being taken away into the woods forever, only to be made into a 'Shake Hands' undead themselves proliferate most folk lore. A more positive view of these undead spirits is that they are simply looking for a friend and very lonely.