The title says it all. This is your stock human grenade. While usually not as useful as the plasma grenade, the frag nonetheless does have its advantages; the main one being that it has a considerably shorter fuse. An excellent device for thinning out large groups of Grunts or dispersing swarms of Jackals, the frag can also be put to devastating effect in an anti-vehicular role. Not quite skilled enough to stick a plasma to that passing Warthog? No problem - flip a frag grenade at it, and as long as you're close, up they go. Observe the usual rules of grenading: i.e. don't throw it at your feet, use rebounds to your advantage instead of against it, etc.
Easy to find in human-controlled areas, scarcer in mainly Covenant ones.
One warning: a frag grenade will not take out a tank. Not in one hit, anyway. Plan accordingly.
Pros: Short fuse, good concussion, an effective vehicle-disabler.
Cons: Not as lethal as plasmas, not as effective against shields, cannot be "attached", difficult to find when in Covenant territories.
Usage: Disrupt vehicles with a well-placed frag, send swarms of Covenant or Flood into the air with a bang. Usually not so wise to use against single enemies.