Shape-Shifters
I believe a shape-shifter is anyone or anything that changes form on will whereas most people see them as any being who changes form. An example of would be the Greek Gods. They can turn into any form they want and always control it. The Greek Gods typically use their shape-shifting abilities to confuse or take advantage of people. Jacob Black, from Twilight, changes form willingly and although Meyers classifies him as a werewolf, Edward and Aro later clarify that he's a shape-shifter.
"These creatures truly have nothing to do with the Children of the Moon. They have merely inherited this skill from their fathers. It's genetic, they do not continue their species by infecting others the way true werewolves do." -Aro, Breaking Dawn
Boggarts
Boggarts, a creature from Harry Potter, area unique form; half shape-shifter, half were-creature. For those of you who don't know what this creature is, they're magical creatures that turn into a person's worst fear. Bogggarts control this change, but lose control after that. Once a witch or wizard has cast the Ridikulus spell, the fearsome image is changed into something funny. For example, Ron Weasley's fear is spiders. When he casts this spell, the giant spider shaped boggart suddenly has roller-skates and he starts slipping around, therefore diminishing Ron's fear.