A grenade is a small explosive device, a little bomb that can be thrown by hand. The earliest grenades were contained in ceramic jars and thrown by ancient Byzantine soldiers.
If you've ever seen a war movie, you may have observed soldiers pulling pins out of hand-held grenades, throwing them, and watching them explode at a distance. The first grenades were extremely dangerous to the people holding them, and modern versions improved the thrower's safety even as they became more deadly for those they were used against. Larger grenades are propelled with launchers, gun-like weapons that are also used in combat.