1337 speak, or 13375p34k, both pronounced "leet speak", is basically hacker talk in English. Or at least, that is how 1337 started out. Letters that are similar in appearance to numbers (capital I and 1, lowercase L and 1, uppercase A and 4) were swapped out for those similar looking numbers in order to get around wordfilters.
Hackers, being incredibly awesome, were imitated by lame idiots who pretend to hack, and who talked in 13375p34k all the time, not just to get around wordfilters.
Specifically about 1337, it originates from a hideous misspelling of "elite" (elite became "leet", which became all its glorious dysfunctional misspellings from l33t to full 1337), most likely propagated by lame idiots. 1337 has over time come to encompass almost any sort of word compression over the internet that is not actually pronounceable in real life "sry", "plz", etc., or misspellings just to be cool "hay", "guyz", etc.
As 1337's popularity is mostly attributed to flaming imbeciles, using it in anything other than a joking or sarcastic manner will make you look like a rather large idiot.
About AI, there's lots of AI related tutorials
here, but most require knowledge of JASS.