This is the language used by many noobs and other people online who are generally uneducated in english. People who cannot type may also use it. It is basically the replacement of actual words with letters or numbers that sound like them, regardless of how short the original word is. It is done supposedly for the sake of speed, but is seen on forums and other things that require no such rush. Though some think doing so on IM is acceptable, this is only so if the person lacks the ability to comfortably type a four-letter word in the time it takes to search out and poke a single key. It also includes the intentional misspelling of words and incorrect grammar and punctuation in order to save the amount of keys you have to press to make the words sound right.
It is also known as "noob talk", "AOL talk", and various other names.
"Are" would be "r"
"You" would be "u".
"Your" or "you're" would be "ur".
"To", "two", or "too" would be "2".
"for" would be "4".
"Before" would be "b4".
A sentence might look like this:
"y r u calin me a noob im not a noob ill tel on u if u keep sayin dat im a noob ur a noob 2 ive been here 4 a lng time b4 u and ill b here after ur gone"