This is the best I could find
* Split end (X or SE): A receiver on the line of scrimmage, necessary to meet the rule requiring seven such players at snap. Where applicable, this receiver is on the opposite side of the tight end. The split end is farthest from center on his side of the field.
* Flanker (Z or FL): A receiver lining up behind the line of scrimmage. Frequently the team's featured receiver, the flanker uses the initial buffer between himself and a defender to avoid jamming, legal contact within five yards of the line of scrimmage. The flanker is generally on the same side of the formation as a tight end. As with the split end, this receiver is the farthest player from center on his side of the field. The flanker is probably lined up just like a split end except that he is just behind the line of scrimmage, being in the backfield and not on the line.
* Slot receiver (Y or SL): A less-formal name given to receivers in addition to split ends and flankers. These receivers line up between the split end/flanker and the linemen. If aligned with a flanker, the slot receiver is usually on the line of scrimmage, and if with a split end, off the line of scrimmage. As with the flanker position, a featured receiver often takes a slot position with a split end to avoid jamming.
* Slot back: A receiver lining up in the offensive back field. Canadian and Arena football allow them to take a running start at the line. They are usually larger players as they need to make catches over the middle. In American football slot backs are typically used in flexbone or other Triple Option offenses while Canadian football uses them in almost all formations.http://en.wikipedia.org/wiki/Wide_receiver