Boos were supposedly based on the wife of a certain Nintendo producer -- meek and shy to your face, vicious and cruel behind your back. This certainly describes the Boos, who have kept this basic personality since their debut in SMB3.
Based on their behavior in games like SM64 and Paper Mario, we can assume that Boos are the spirits of deceased residents of the Mushroom Kingdom. Exactly who these ghosts were before they died has yet to be revealed.
In Luigi's Mansion we finally meet the King of the Boos, a minion of Bowser with a vast army of undead creatures.
New Super Mario Bros. (NDS)
Super Mario 64 (N64)
Super Mario All-Stars (SNES) [known as "Boo Diddley"]
Super Mario Bros. 3 (NES) [known as "Boo Diddley"]
Super Mario RPG: Legend of the Seven Stars (SNES) [known as "The Big Boo"]
Super Mario World (SNES) [known as "Boo Buddy"]
Yoshi's Story (N64)
Mario Party 3 (N64)
Yoshi (NES) [known as "Boo Buddy"]
Yoshi's Cookie (GB)
Yoshi's Cookie (NES)
Yoshi's Cookie (SNES)
Mario Kart 64 (N64)
Mario Kart DS (NDS)
Super Mario Kart (SNES) [known as "Ghost"]