Defensive End

At the outside edge of the defensive line are two guys, the defensive ends. Unlike the tackles, who have a job that's very complicated, the defensive ends have a relatively simple job. If it's a running play, their job is to move forward a step or two and make sure the running back cannot get outside of them, where he will have to be chased down by cornerbacks and safeties. They want to make sure the running back is funneled inside to the big crowd of offensive and defensive linemen and linebackers, where he will be slowed down and quickly stopped.
If it's a passing play, the defensive end has the job of getting to the quarterback as quickly as possible and doing anything and everything to disrupt him. Their favorite thing to do is to swat the football out of the quarterback's hands, causing a fumble, then pick up the ball and run for a touchdown. This is very rare, and when it happens it makes the TV highlights for the entire week, and it will also make the team year-end highlight video. Their next favorite thing to do is tackle the quarterback, which is called a sack. Sacks are very good things for the defense. Their next favorite thing to do is to try to jump up at the last second, wave their arms about, and hit the passed football as it flies by, turning a pass into a free-for-all. In short, if it's a passing play the defensive end's job is to create chaos.
Defensive ends must be very fast, so they tend to be somewhat light for a lineman - often 245 to 260 pounds. The most famous defensive end of all time is Reggie White, who weighed about 300 pounds and was incredibly strong and fast. Reggie set the NFL record for most sacks in his career. To this day, many team general managers say if they were to build a defense from scratch, they would want to start with Reggie White, who is regarded by many as the best defensive player ever.
Defensive ends who are good at sacking quarterbacks, getting 10 or more sacks per year, are extremely rare and very valuable.