Wide Receiver

There are a few guys on the football team who have the job of running at near Olympic speeds and catching passes thrown by the quarterback. These guys are called receivers, or wide receivers, or split ends, or flankers, or slot receivers. They are anywhere from 5'7" to 6'6", but because they have to be fast they are normally light by football standards, 175 to 215 pounds. Unfortunately, if there's a guy on your football team who has enough ego for any five guys, he's almost certainly a receiver. There are a certain number of receivers in the NFL who seem to delight in displaying outrageous behavior almost weekly.
The rules of football say that the offense must have at least seven guys lined up even with the football. The five offensive linemen and the tight end make six, so we need another. In the diagram above, the seventh is labeled SE, for Split End. He lined up like a Tight End, but on the other end of the line. However, he's not lined up tight against the tackle, but rather split off to one side, hence Split End instead of Tight End. The Split End is also called the X receiver. He's also called a wide receiver, because he lines up wide to one side.
The split end is sent way out to the side of the field for several reasons. It gets him away from the fat guys, so he's not very likely to get seriously beat up. It makes the defense watch a wider portion of the field, which helps open up the center for running lanes. And, if your running back is a cut back sort, the split end is in a good position to block one of the safeties. He can run out as if he was going to catch a ball in the middle of the field, then suddenly turn on the safety and block him away from the running back. On nearly every long run you will see a receiver throw a key block to open up the field for the running back.
The receivers have "routes" to run. This means on any given play, the receiver has a particular place to run to. They don't just run around at random. These routes are important because the quarterback also know the routes, and therefore knows where the receiver is suppose to be. This allows the quarterback to find a receiver quickly. Also, on a given play, the quarterback will be using a particular number of steps in his drop back. He knows from practice where in the route each receiver is after he has dropped back his pre- determined steps. So the quarterback knows from the play where in the field the receiver is supposed to be running, and from the timing of his steps and the receivers running he knows exactly where each receiver should be.
In the diagram above, the SE is being covered by a cornerback, labeled CB on the diagram. The cornerback covering the split end is lined up a little to the outside of the split end. This tells the quarterback and the receiver that this cornerback is going to stay to the outside of the receiver and try to force him in towards the center of the field where the safety can help cover him. This line up tends to show the defense is in zone coverage on that side of the field, meaning that defenders on that side of the field are responsible for areas, not for particular players. That tells the quarterback that the free safety and the cornerback on the left side of the field are not likely to come running up to try to sneak in and ambush him. It also tells him his split end is going to be covered by two guys and is going to have a hard time getting free to catch the ball.
The next receiver on the team is the tight end, who is also called the Y receiver. We've already covered him elsewhere.
The third receiver is called the flanker or the slot receiver. With the five offensive linemen, the tight end, and the split end all lined up even with the football, the requirement for seven guys lined up is filled. The flanker gets to line up a few feet back from the line. This means that the cornerback covering the flanker cannot hit him as soon as the ball is snapped - he has to catch him first. This gives the flanker a big advantage. Generally, the flanker will be a bit smaller and faster than the split end, as the split end has to be prepared to be hit the instant the ball is snapped. The Flanker is also sometimes called the Z receiver.
The flanker is going to get to catch a lot of passes because he's quick and fast, and because he's lined up on the quarterback's right side. Right handed quarterbacks prefer to throw to their right, instead of throwing across their body to the left.
The flanker also has responsibilities to block defenders if this will be a running play. He also helps just by being split out to the right side - this gives the defense more field to worry about, and who ever is assigned to cover the flanker is one less guy available to try to sneak into the backfield and ambush the quarterback.
In the diagram above, the cornerback "CB" covering the flanker "FL" is lined up a little to the inside of the flanker. This tells the quarterback and the flanker that the cornerback is going to try to keep the flanker from running in towards the middle of the field, instead the cornerback is going to try to direct the flanker towards the out of bounds line on the edge of the field. The receiver may not touch the out of bounds line - if he does, he's no longer allowed to catch the ball. So the out of bounds line is the cornerback's friend. By lining up inside the flanker, the cornerback is showing that he wants to use the out of bounds line as his friend, which indicates to the quarterback and the flanker that this cornerback is in man coverage. This cornerback is indicating that he's going to run along side the flanker where ever he goes. It also means that the cornerback does not expect the strong safety SS is going to help him. This tells the quarterback that the flanker is in single coverage, not double covered. It also tells the quarterback that the strong safety apparently has some other job to do, either stay in the middle of the field to help out with the tight end or running back. Or possibly the strong safety is going to try to sneak into the back field and ambush the quarterback.
If the flanker lines up between the split end and the left tackle, then he's called a slot back. He's the same guy and has the same responsibility, they just change his name to confuse people.
Most frequently, the X receiver or SE will be the largest and fastest receiver on the team. He will specialize in running long routes, threatening a long pass. The Flanker will be a smaller but quicker player who will specialize in shorter routes, often crossing over the middle of the field.