There are several different types of grenades. AP are the most common, they throw fragments of steel in a significant radius and maim or kill quite effectively. Then there's flash-bangs, which are intense light and percussion to blind, deafen, and disorient immediately before a breach. There's HE grenades, which are meant for destroying light vehicles, equipment, and sometimes bunkers. I guess that's what movie grenades are supposed to represent. But the worst would have to be Willy Petes - White Phosphorous incendiary grenades.