Just a brief description of the X Y Z Receivers. Maybe someone else can go into more depth and apply it to a couple teams for examples?

X Receiver: The X Receiver is also known as the split end. This receiver lines up on the line of scrimmage. Must be a strong receiver as its a little more difficult to get off the line and into his route tree. Usually in press coverage.

Y Receiver: The slot receiver: Usually one of the teams best route runners. Very quick and very good hands. This guy lines up on the inside of the formations. Depending on the formation, the TE could assume this role either from a stand up position or from a 3-point stance in something like an 11 personnel (1 RB, 1TE). Wes Welker may be the most famous Y receiver.

Z Receiver: The Flanker usually lines up off the line of scrimmage to avoid the press coverage from the defense. This player must be quick into his routes. He is also usually your deep threat or your screen guy...more so than your X Receiver.

All this will vary depending on what formations and what personnel a team will put on the field...but above is just a quick, basic description of what each receiver position (XYZ) is.

Hope that helps?