Guard (Grappling)
The guard as it's called in grappling disiplines such as Brazilian Jiu jitsu or Sambo; is where one lies on his back
holding his or her opponent's trunk with there legs. 
From this position it is possible to attack your opponent 
with joint locks, chokes,strikes or sweeps/reversals. 
It also is possible for the person in the top position to effectivley strike inside the guard.
If you are taken to the ground with your opponent coming down on top of you. It might be best to assume a type of guard, instead of trying to turn your back to him or allowing him to fully mount you.

My opponent connects with a right hook knocking me off balance. Seizing his chance he charges and tackles me in an attempt to take me down and ground N pound me. 
As we hit the ground I Close my legs tightly around his hips thus forming a closed Guard (Grappling). I cup the crown of his head with one arm and under hook his arm with my other. I suck him in close to me. By breaking down his posture I can better inhibit (But not take away) his ability to strike than if he had me fully mounted. 