According to urban dictionary:

Commonly known as Leet language (1337)
1337 h4x0r Translates to Leet Haxor (hacker with a kick)
In the language 1337 4 replaces A and 0 commonly replaces o.Tsutomu Strikes Again!
Pirate: OMFG you newb i just owned your Sorry ***
Ninja: Ha.. it is you who is the newb! forgot about the Language Filter didnt you Newblet!?
Pirate: Phuck J00
Ninja: :0 you h4x0r
Pirate: 1337 h4x0r j00 M34N!
Ninja: … i hate you …