Guard (American football)

On each side of the center are the guards, which are unimaginatively called the left guard and the right guard. These guys have two different possible jobs to do. If the quarterback has called a pass play, where he will try to throw the ball over a mess of very large guys who are all wrestling and hit a speedy receiver in full flight, then the job of the guards is to be large impenetrable walls and absorb any punishment the defensive guys care to put out. The guards are to form a wall with the center that the quarterback can stand behind, unmolested, while the quarterback tries to get the ball to the right person. If the football team spends most of their time passing the ball, the guards will tend to be very big guys, weighing perhaps as much as 325 to 330 pounds. These very heavy guys can't move as fast as somewhat smaller guys, but they can absorb a lot of punishment.
If the quarterback has called a running play, where one of the players will try to run with the football, the guards have their second job. In this case their job is to bull rush whoever is ahead of them and create a hole in the defense for the runner. The guards which are best at bullying a hole in the defense are just a bit smaller, perhaps 300 to 310 pounds, and a bit faster and more athletic. It's a very different job to run forwards a few steps while pushing a 320 pound defender than to stand more or less in one place and take whatever the 320 pound defender dishes out without moving or flinching.
We call these two jobs pass blocking and run blocking. Pass blocking is the job of standing still and not letting anyone through or by. Run blocking is the job of leaping out and trying to put a defender on his butt.
If the quarterback is a relatively immobile guy, then the guards must be very large "wide body" models to be able to stand and absorb punishment, as the quarterback is not going to succeed if he has to run for his life. If the quarterback is a faster guy, then the team can use slightly slimmer, faster, more athletic guards. These guards will be better at run blocking, but will be just a little less reliable in pass blocking. However, if someone gets past the guards the assumption is that the mobile quarterback can run a few steps and get away for a couple of seconds.
Most teams think that it's relatively easy to replace guards, so like centers these are not the highest paid guys on the team. You would expect a guard to be paid $1M to $3M per year.