A Boo is a ghastly enemy that haunt multiple locations in the Super Mario series. Usually appearing in mansion levels and scary themed worlds, Boos are a harder enemy to overcome, with most Items not being strong enough to defeat them. They frequently act in mysterious ways, such as sneaking up on you when you have your back turned (New Super Mario Bros. Wii), or popping out of nowhere.
The "Ghost" Item, an early version of the regular Boo, was introduced in Super Mario Kart's battle mode. Boos had the ability to turn the user invisible and steal an opponents Item (if they had one). Skipping though Mario Kart games, all with the same affect, the Boo Item saw its last appearance in Mario Kart DS.SNES Ghost Valley 2 grants Boos a cameo in Mario Kart Wii. They float around throughout pretty much the track, but don't do any harm at all. Maybe they're intimidation or something.