1337 = L33T = LEET = Elite.
1337 Language represents writing in symbols instead of letters. For example, but not limited to:
1 = I / L
3 = E
4 = A
5 = S
6 = SH (in some languages)
7 = T
0 = O
+ = T
|_ = L
\/ = V
etc.
You can notice some l33t sp34k in
Linkin Park - Reanimation album song titles
(Frgt/10; P5hng Me A*wy; Plc.4 Mie Haed)
Megatokyo comic strip
(3Y3 |\|33d j00 t0 g47z d4 d0c70r. 3Y3 g07z s0m3 b4d p4|n 1|\| m4 ch4s7, | n33d m4 p|11z!)
and many many more.