In an ideal pass, the quarterback will throw the ball down the field and a wide receiver or tight end will catch it for a gain. The end!
Unfortunately, that's not always the case. Sometimes, things go awry... Here are some of the other situations that might happen when the QB attempts a pass.
What is an incomplete pass? When the quarterback attempts to throw the ball but it hits the ground, goes out of bounds, or a player misses the catch, that's called an incomplete pass. Once a pass is ruled incomplete, then the game clock stops and the next down starts at the same place as the last possession. Basically, an incomplete pass burns a down.
What is intentional grounding? Sometimes when the QB throws the ball and no one catches it, the refs will call the penalty of "intentional grounding." Intentional grounding looks similar to an incomplete pass, but when the QB throws the football there are no receivers anywhere in the general vicinity. Most of the time, a QB will do this when he’s about to get sacked. Instead of going down and taking negative yards on the play, he’ll desperately try to throw to someone on his team, into the dirt, or out of bounds. If this penalty didn’t exist, the missed throw would just be an “incomplete pass,” and the next down would start at the same place. But with the penalty, the offense loses a down and has to start its next possession 10 yards back.
What can the QB do if he can't intentionally ground the ball? If a QB is facing a lot of pressure from the defense, then he has a couple of options to avoid getting sacked: (1) he can either slide onto the ground and the next down will start where he landed; or (2) he can throw the ball away. What is throwing the ball away? Throwing the ball away looks a lot like an incomplete pass or intentional grounding because the QB is throwing the ball out of bounds, which can make it a little confusing. The big difference is that QB must run outside the pocket, aka to the left or right along the line of scrimmage, beyond the area where his offensive line defends him. Then he can throw the ball out of bounds, as long as he throws it beyond the line of scrimmage. Just like in an incomplete pass, the game clock stops and the next down starts at the same place as the last possession.
What is a fumble? A fumble happens when any player, who has possession of the football, drops it or gets it knocked out of his hands. Then the ball is considered "live" meaning it is still in play. Both sides will dive for the ball and try and get possession of it. That's why you see those big pile ups of players scratching and clawing until the refs pull them away to see who came away with the football. If the other team gains possession that's called a turnover.
Usually a fumble will happen to a running back, receiver or tight end, but sometimes it can happen to a quarterback. If the QB is in the process of throwing the ball, but it gets knocked out of his hands as he's making a forward motion, that's considered an incomplete pass. But if the ball gets knocked out while the QB's arm is still moving in its backswing, then it's a fumble.
What is an interception? If the QB throws the ball but a player from the defense catches it before it hits the ground that's called an interception.