BB Guns - either co2 powered ( like paintball guns) or cocked each shot . BB's are metal , and can easily go through tin cans , hence can easily go through skin. Or kill you if shot in the right spot ( neck or something )
Airsoft - Usually spring loaded or electric , shoots plastic BBs , and wil generally leave small bruises or welts , can draw blood if shot close enough with the right gun.
Which should you buy ? if you want target practice in your backyard , or to kill birds and squirrels , BB gun. If you want to shoot your friends in the leg if they get out of line , an airsoft gun.