|Wikipedia explains it all for you.
||[Nov. 17th, 2006|08:28 am]
한국 사람이 아니다
Good lord. What would we do without Wiki?
Not only do they have a lengthy entry on the O RLY? owl and "teh", but there's also the definition of the now-infamous "I'm in ur (whatever), (whateverin') ur (whateverz)!"
It's on a list of internet phenomena, along with almost every internet phenomenon I can think of (although, oddly, it doesn't include that Peter Pan Guy in Florida).
It says: IM IN UR BASE KILLIN UR D00DZ — Originally thought to have been posted on a gaming forum showing a member of one gaming faction killing members of a rival faction, with the caption split into two phrases at the top and bottom of the image. Many subsequent versions have been created, all following the "I'M IN UR (noun)(verb)ING UR (noun)" format.
So there you go. For some reason, it seems to be popular to put together with cat photos; I've seen most of the parodies on the cat communities I belong to.
This is also pretty useful: List of Internet Slang Phrases. Go forth and forward to all your parents!