OK:(posted 9142 days ago)Frag grenades: Normally, troops that are in cover always strike first in close combat. Frag grenades stop this, so that you fight simulatneously.
Krak grenades: Can only be used against vehicles and walkers. Roll to hit as normal, exchanging all your attacks for a single attack with a krak grenade. Basically, they allow you to make a Strength 6 attack against a vehicle.
Hope this helps!