It's a 9 digit number that US bank accounts have. It basically tells a system where to route the money to. Then the account number is used to identify the specific account.
Yes. Non US banks use a variety of other routing codes. The most common are BSB, and IBAN. http://en.wikipedia.org/wiki/Bank_code
But you still can open US bank account from Pakistan and can withdraw PayPal fund from Pakistan with that bank's debit card. If you need further help about it, let me know.