X – Split end, the wide receiver that lines up on the offensive line.
Y – Tight end.
Z – Flanker, the wide receiver off the line, the guy in motion. Slot receiver who is usually smaller and faster.
E – A third receiver in the formation.
H-back – Tight end in motion who slides to the backfield and blocks.
U-back – The third tight end in a three-tight end formation, in an obvious run situation. This guy is just a blocker.
Thankfully on the offensive line, the positions are clear-cut. "We don't flip flop our guys so a guard is a guard is a guard, a tackle is a tackle is a tackle," said Packers line coach Larry Beightol.
Defensively, the names get more complicated and a little more creative:
Elephant end – Defensive lineman on the right end who is the designated pass rusher. Contrary to what the name suggests, this guy is the quickest and most nimble of the linemen.
Power end – Defensive lineman on the left end who is bigger and stronger than his counterpart.
Mike or Mac – Middle linebacker.
Will – Inside linebacker who lines up on the weak side, or opposite of the side where the tight end lines up.
Buck, Sam – Linebacker who defends the side where the tight end is lined up. This is called the strong side.
Lurk – The linebacker or extra cornerback who plays in the nickel formation, a defense that adds an extra man to the secondary.
According to one theory, the linebacker names stem from simple military language: "mike" for middle, "will" for weak and "sam" for strong.