Slang in "1337 speak", which is a language used by people chatting online.
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)

The safeguarding of Earth and other worlds from biological cross-contamination (i.e. billionaires with too much time on their hands).

Also known as “planetary protection.” Planetary protection / quarantine “reflects both the unknown nature of the space environment and the desire of the scientific community to preserve the pristine nature of celestial bodies until they can be studied in detail.”

There are two types of interplanetary contamination. Forward contamination is the transfer of viable organisms from Earth to another celestial body. Back contamination is the transfer of extraterrestrial organisms, if such exist, back to the Earth's biosphere.

1337 speak for the word "you".

1337-speak replacement for "you". Usage is correct when used to replace the "-ed" of a past tense verb followed by "you". Commonly misused as a simple replacement for "you" regardless of the preceding word.

1337 version of you