Boos are ghost enemies that attack Mario only when he is not looking at them. They can pass through walls, obstacles, basically anything, and the only way to kill them is with a Super Star or a Goomba's Shoe. If a boo has wings, it will wait a bit before attacking Mario, but moves faster.
If you shake them, they will become Boo Buddies, a circle of seven boos in a pattern that could fit eight but there is one missing. They constantly circle around, and if they have wings, they will cycle in and out. If they are in a Lakitu's cloud or a Koopa Clown Car, they will control the vehicle from around it and leave when Mario gets in it.
If you put it on the ground it turns into a Stretch. Stretches move along the ground and pop in and out.