The etymoligical roots of j00 are quite unclear, yet it seems that it is derived from accents of the english that are found humourous by some and made into a joke which soon developed into norm. Such is the history of many internet slangs.
-- GRAMMATICAL USAGE --
j00 should NEVER be used as a subject, only as an object or within a prepositional phrase. If needed to be used as a subject, it should ALWAYS be replaced with "you" or another 1337 speak equivilant.
(ie. "Are j00 there?" is incorrect. "I pwn j00" is correct.)
j00 is often used in conjuction with mathematical signs of comparism, such as "=" (equals), ">" (greater than), or "<" (less than). Again, in these cases, j00 is the object and is always on the right side of the sign. There are some cases where it is okay to do otherwise, as this rule is somewhat lenient. ">" is most frequently used, and is read as "owns"
(ie "Rockets > j00" - read as "rockets pwn you", "j00 = stupid" - special case)
giant sticks of dynomite > j00
I h4x0rd you -> I h4x0r j00
I pwnd you -> I pwn j00
However, the following examples are both incorrect:
I pwn you (present tense pwn) -> I pwn j00 (tense changes)
I pwnd you -> I pwnd j00 (redundant)
(2)1337 speak for "you"