In American football, there are plenty of ways to get hassled by the officials. If you don't line up in the right spot, you team gets penalized. In this lecture, we'll cover one such penalty: intentional grounding. Action!
Immediately after the snap of a normal play, defensive linemen try to rush forward, through the offensive linemen, and discuss classic philosophy with the quarterback. If many topics will be covered, a safety or multiple safeties may follow the linemen in and take notes. Or just grind his bones into the ground.
This happens in professional and college football. As the defensive players push forward, the offensive linemen and any additional blockers tend to collapse into an arc in front of the quarterback. This formation is aptly called the pocket. If the offensive line holds, the quarterback is safe in the pocket. If the line breaks, the quarterback is considered to be under pressure and he must either scramble or unload the ball in a hurry. Or brace for the bone grinding.
If the quarterback does not want to lose the yardage or give up the points to a (different type of) safety, he may try to throw the ball. Some quarterbacks can handle pressure very well and stay cool and collected. Other quarterbacks do not react well at all and sometimes become so panicked that they are very likely to throw an interception. Now let's Tivo this situation up a notch.
If the quarterback throws the ball away to an empty bit of field where nobody is standing, it's intentional grounding as long as he is in the pocket. Intentional grounding only counts when the quarterback throws the ball away to avoid getting tackled.
If he is out of the pocket and under pressure, the rules are more lenient. While under pressure, as long as the quarterback throws ahead of the line of scrimmage, it is not intentional grounding, regardless of where the ball lands. This cuts the quarterback some slack, considering he may have to throw the ball while running, or use his non-dominant hand, etc. It can get hectic out there, so let's play nice.
Part of playing nice is admitting when you screw up. If the quarterback threw the ball to avoid getting tackled and there was no reasonable chance of it being caught, you get a nasty penalty. In college, it's loss of a down and placing the ball where the grounding occurred. In professional football, it's a loss of a down and 15 yards in the wrong direction.
A derivative of this is spiking the ball. If a team uses all of their time outs for the half, they have no way to stop the clock without penalty. If the quarterback takes the snap and immediately slams the ball to the ground, he has spiked the ball and the play ends. This has the unfortunate consequence of burning a down but it does stop the clock. Sometimes, it's just that important.
P.S. Go Dawgs!