31337 was originally created by hackers who used UDP port 31337 to hack into computers. 31337, when translated into letters, coincidentally became "eleet", which sounded very much like "elite". And so they called themselves 31337, or elite hackers.

This "word" was later to be shortened to "1337", or "leet". The use of this word has spread to the gaming community, where many no-life-still-thinking-they-are-cool people use it. It has also become a language, where symbols are used to resemble letters.

People who type in 1337 are usually "high ranked" people with no life other than playing their favorite MMORPG.

They would say something like:
OMFG!! 1 pwnt da n00b bad so STFU NUB!!

Translation:
Oh, my fucking god! I owned the newbie bad, so shut the fuck up noob!

This, as you can see, makes no sense. Except if it's to make fun of people to whom it does make sense.

Hacker "Sp33k" for leet, or elite. Originating from 31337 "eleet", the UDP port used by Dead Cow Cult, a hacker group, to access Windows 95 using Back Orifice, a notorious hacking program.

n. 1. Nestled between 1336 and 1338, 1337 is commonly known as a number, or for the more advanced; an "integer"
2. Often misused to satisfy a lamen's ego, the self-proclaimed 1337 will misuse the title, pretending to hold Xtr3m H4x0r capabilties while in fact masturbating to preteen beastiality at 2:00 AM with baggy eyes; anonymously hassling n00b's with better things to do than kill their babies over mindrot such as Everquest and Warcraft. See also "colon debris" and "queefstain"

A language in which numbers and symbols are put together to look like letters. Some people create their own 1337 letters and it makes them look more 1337 by fellow 1337-speakers. Here is an alphabet of 1337 letters I know and have created:

A: 4 or l\ or ^ or @ or /\ or /-\
B. l3 or 8 or ß or ]3 or l:
C: ( or < or © or ¢
D: l) or l> or ])
E:3 or £
F: l= or # or ƒ
G:6 or 9
H: # or l-l or (-) or !-! or }-{ or }{ or l+l or )+( or !+! or }+{
L: 1 or ! or ]
J: _l or _/
K: l< or l( or l{ or l<=
L: l_ or ! or 1
M: l\/l or /\/\ or l\l\ or ^^
N: l/l or /\/
O: 0 or () or <> or * or ø or Ó or °
P: l* or l> or |D or l^ or l+
Q:& or (\) or ¶
R: l2 or ®
S: 5 or $ or §
T:+ or 7
U: l_l or /_/
V: \/
W:|/\| or \/\/ or |/\/ or \/\|
X: >< or }{ or :-:
Y: ¥
Z: 2

one thousand three hundred and thirty seven

(i do know the true meaning)

Once used by the hacker community as sort of a sub-culture language, ANYONE using this type of speak in the present day in age is either:

A) A clueless net newbie who somehow thinks its still "cool" to speak this way.

B) A veteren net user who uses it to parody the type of people mentioned in example A.

1337 is the language of hackers and newbies across the internet. Also, the words in 1337 are usually short(ie "are" becomes "r") and intentionally spelt wrong(ie "the" becomes "teh", this is most common with e switching places and o's becoming p's)

the mostly complete translation chart(exluding unicode characters, ie ©):
A=4 /-\ @ ^ /\
B=8 ]3 ]8 |3 |8
C=( {
D=) } |) |} |> >
E=3
F*=# |=
G=6 9 (_>
H=# |-| (-) )-( }{ {-} /-/ \-\
I=1 ! |
J**=_| j
K=|< |{
L=1 |_
M=/\/\ |\/| \/
N=/\/ |\| \
O=0 ()
P=|D |* |>
Q=0,
R=|2 |? |-
S=5
T=7 + ']'
U=(_) |_| \_\ /_/
V=\/
W=\/\/ |/\| /\
X**=>< }{ X
Y='/ %
Z**...

1337. Originally used by hackers to avoid simple net searches that would turn up, for example, "I am an elite hacker".

Instead hackers would say "1 4|\/| 4|\| 31337 |-|4x0|2".

"31337" was later shortened to "1337".

Now 1337 used mainly by the gaming community (also in chatrooms) to mean an especially skilled player.