I call it "being an idiot."
Some of the reasons people use this form of grammar and spelling is because of social networks, such as Twitter, which only allow a certain amount of characters.
Such words might include:
Dis- That
Dat- That
Da- The
Luv-Love
Wuv- Love
Plz- Please
Tank-Thank
and so on.

What drives me ABSOLUTELY CRAZY is when people use words such as:
Lyke- Like
Iz- Is
Wuz- Was
Luff- Love
Pwease- Please
Shank- Thank
Meh- Me
and so on.

There is no reason not to use the correct form of these words. I believe that some people use this form of typing/speaking to look "cool." Ignorance is not attractive.

This is coming from a 13 year-old, so if you believe that all teenagers nowadays talk like this, that is not true…

P.S. I believe this form of writing is called "text talk."