From Warhammer 40k - Lexicanum
Imperial Plasma Grenades make use of a deliberate plasma containment failure to unleash a highly lethal blast of heat and light similar to a miniature sun which can take out almost any target.
The Eldar and Dark Eldar use plasma grenades in placement of the more crudely designed Frag Grenades used by the Imperium. They use a small amount of exploding plasma to blind the enemy and prevent them from properly using their terrain to defend their position against advancing enemy forces.
- 1: Rogue Trader Rulebook, pg. 126