BBs are the ammunition used in BB guns. They are tiny, lead shot projectiles in the shape of a sphere. The name BB comes from the size of the ammunition, which is usually .177 caliber or 4.5 millimeter. The sizes of lead shot used in shotgun shells were B, BB, and BBB. This middle sized piece of ammunition in what gave the gun its name.
BB guns also use another type of ammunition, called pellets. Pellets are shaped pieces of lead, sometimes with a point at one end, sometimes hollow tipped. They generally have a flared back so as to engage the grooves inside the barrel of the gun and give stability when fired. Another type of ammo is the dart. These are especially dangerous, as they have a sharp tipped end, much like the darts which are hand tossed in the game of darts. Darts in BB guns are generally used for target shooting only, and again are especially dangerous.
Contrary to popular belief, BB does not stand for "ball bearing," "Baby bullet," or anything else. It refers simply to the size of the ammo used in these weapons.