Leet (sometimes written as "1337" or "l33t"), also known as eleet or leet-speak, is another alphabet for the English language that is used mostly on the internet. It uses various combinations of ASCII characters to replace Latinate letters. For example, leet spellings of the word leet include 1337 and l33t; eleet may be spelled 31337 or 3l33t. It is used on the internet in forums, chat rooms and online games.

Leet is mostly used for English, but can also be used with other languages including French, Spanish and German.

Leet-speak 
Leet-speak is a mixture of words (mostly computer-related jargon) spelled incorrectly intentionally*, usually coming from typographical errors (e.g. the becomes t3h). The words of Leet-speak are usually put together to create a dialect (small language). This dialect is used in some places for funniness. l33tspeak uses numbers, ASCII symbols, and diacritics together to make symbols that look like Latin letters. For example, in the name "David" would translate in 1337 as |)/-\\/1|).

1337 Alphabet Examples 
Most of the time it is only the vowels that are changed and then usually only they are replaced by numbers; therefore, it is unusual to see someone use the leet alphabet to spell out a whole word (although there are common exceptions, namely 1337).

These are only examples; these are not mandatory for Leet-speak, which is not officially defined anywhere, like most slangs.
 A: ∆,4, /-\, /_\, @, /\, Д, а
 B: 8, |3, 13, |}, |:, |8, 18, 6, |B, |8, lo, |o, j3, ß, в, ь
 C: <, {, [, (, ©, ¢, с
 D: |), |}, |], |>
 E: 3, £, ₤, €, е
 F: 7, |=, ph, |#, |", ƒ
 G: 9,[, -, [+, 6, C-
 H: #, 4, |-|, [-], {-}, }-{, }{, |=|, [=], {=}, /-/, (-), )-(, :-:, I+I, н
 I: 1, |, !, 9
 J: √,_|, _/, _7, 9, _), _], _}
 K: |<, 1<, l<, |{, l{
 L: |_, |, 1, ][
 M: 44, |\/|, ^^, /\/\, /X\, []\/][, []V[], ][\\//][, (V), //., .\\, N\, м
 N: |\|, /\/, /V, ][\\][, И, и, п
 O: 0, (), [], {}, <>, Ø, oh, Θ, о, ө
 P: |o, |O, |>, |*, |°, |D, /o, []D, |7, р
 Q: O_, 9, (,), 0, kw
 R: |2, 12, .-, |^, l2, Я, ®
 S: 5, $, §
 T: 7, +, 7`, '|', `|`, ~|~, -|-, '][', т
 U: |_|, \_\, /_/, \_/, (_), [_], {_}
 V: \/
 W: \/\/, (/\), \^/, |/\|, \X/, \\', '//, VV, \_|_/, \\//\\//, Ш, 2u, \V/
 X: ×,%, *, ><, }{, )(, Ж
 Y: `/, ¥, \|/, Ч, ү, у
 Z: 5, 7_, >_, (/)

Letters for numbers 
0 = O or D or space
 1 = I or L or 7 or | or !
 2 = Z
 3 = E or e or m or w or ω or ∈ or ε or ∩∩
 4 = h or A or y
 5 = S
 6 = b or G
 7 = T or j or L
 8 = B or X
 9 = g or J
10 = I and O or L and O or 7 and O
11 = N 
0  =  C or O or D or Θ or o

Intentional misspelling* 
When people write in leet, they spell some words wrong all the time. They might do this to laugh at new people who do not write words with their keyboards well, might just be making fun of English's pronunciation or spelling .

 bai or bi --- bye
 nd --- and
 d00d --- dude
 dog8 --- dog
 form --- from
 gurl or grill --- girl
 guise --- guys
 h4x or h4x0rz --- hacks (see vocabulary entry hax)
 hai --- hi
 j00 --- you
 kewl --- cool
kk - ok
 liek --- like
 lopl --- lol
 smat --- smart
 m8 --- mate
 m2 --- me too
 mastah --- master
 max0r --- max. or maximum
 OMG --- oh my god (variations such as OMGG are also common) OMD ~ (Oh my Dance)
 own or pwn --- see vocabulary entry own
 PMGZ--- OMG with a p and a z
 ZPMG--- OMG originally rushed. Typo P instead of O and hitting Z when shift is pressed.
 OLO --- LOL (LOL stands for laugh out loud)
 ph34 --- fear
 pl0x --- please
 powwah --- power
 pr0n --- porn
 t3h --- the (see vocabulary entry: "teh")
 wen --- when
 wat/whut/wut --- what
 winnar --- winner
 u --- you
 r --- are
 y --- why
 yass/yus/yos --- yes
 ya/yea --- yeah
 cya --- see you
 10/3u --- loveu
wamen- women
what3v

Leet vocabulary 
 leet, l33t, l337, 1337, 31337 --- elite
 l@ym --- laughing at your momma
 ftw, 4tw --- for the win,when something is successful or desirable.
 hax, h4x --- hacks
 haxor, hax0r, h4x0r, haxzor --- "hacker", cheater (note: also often spelled with two x's)
 joo, j00 --- you
 meh m3h --- can mean me or oh well. Used to show undecidedness, unworriedness or often as a sign that the user has nothing to say
 newbie, noob, n00b, n00b13, nooblet, nub, newb, nubbins --- new, newcomer or new (player), used more often as an insult. Thought to originate from new boy, new-blood, or new in business.
 boon or b00n --- more derogatory term than noob with the same insult intended.
 own, ownage, 0wn, pwn, pwnage --- own means to be superior or better than someone, the past tense is "pwnt"
 phear, ph34r, ph33r --- fear
 rox0r, r0xorz --- rock, to be amazing or excellent
 c3n50red --- censored
 teh, t3h --- "the"
 wuut/wuu2 - Basically saying 'What are you up to'. People do choose to say this quite often
 woot, w00t --- Thought to originate from We Own the Other Team. It is used to show celebration or happiness.
 ZoMG!!!11! --- "Oh My God"
 P00nd- Another use of pwn, not to be confused with pr0n.
 Pr0n- Porn (pornography)
 QQ - cry/complain (eyes with tears) or the shortcut to close Warcraft II
 L2<blank> - Learn to <blank>. Often used as L2P (learn to play).
 O RLY - "oh, really?" often responded to by YARLY. Also can be accompanied by an ASCII art image of an owl.
 YARLY - "yeah, really!" often used as response to ORLY
 NOWAI - "no way!" often used as a response to YARLY
 pew pew - shooting, often used in the phrase more pew pew less QQ, meaning, more shooting less crying/complaining
 GG - good game, this can also be used in a condenscending manner.
 BG - bad game, in World of Warcraft this also means Battleground.
 noobcannon, N00BC4NN0N - Noobtube, a weapon in a video game that is either over-powered, or requires little skill to use.
 PWN4G3 N008 - Pwnage noob.
 Stronk - Strong.
 GL - Good Luck, So giving someone good luck before a game
 HF - Have Fun, telling people to enjoy the game
 n1 - Nice One
 ns - Nice Shot; complimenting someone, usually on their aim in a FPS (First-person shooter) game.
 bm - Bad Manners; usually used to call someone out after a show of bad sportsmanship.

Wrong grammar* 
When a person writes in English the person must follow English rules, but most of the time people who write in leet do not use these rules. When they write in leet they try to write it in a special way, so that it is very different from English.

"l33t is teh suck!" (meaning: 'leet sucks') shows that in leet people can use their own rules, and not use the English rules. A lot of times adjectives (describing words) are turned into nouns. The leet word "teh" (the) is also often placed in front of verbs, as in "I teh pwnd j00" (meaning I the owned you, meaning: I beat you badly). another example might be I 4m t3h pwnage!!!!11!!11 (meaning I am the best!)

Leet speakers may also speak normally but without the vowels. Fr xmpl nmbr 1 u cn tll it wrks.= For example, number one you can tell it works.

Pronunciation 
Because some words are too long for use in games they have been written how they sound. Here's a quick guide:
 pwn --- own, pawn or powne
 x or xx (as in haxxorz or haxorz) --- an x followed by a z. In this example, hackzorz would be pronunciation
 R0fz --- rolling on floor screaming, pronounced rawfs

Variations
There are some that are uber 1337...using nothing but symbols and numbers to represent their letters...
Example as follows;

1 4/\/\ 7|-|3 |<33/o3|2 ()|" 7|-|3 \_//\/1\/3|253 (I am the keeper of the universe)

Other websites

 Noslang.com, internet slang translator
 Albinoblacksheep.com, another leet to English translator

*Much of the intentional misspelling said to be part of 1337 is actually more properly part of chatspeak, a completely different thing.

References 

Internet
Slang
Encodings