"u" for "you" is only for texting English. It isn't real English at all. I'm surprised that so many English language learners think that it is a real English word. Frankly, I think it looks terrible in posts (along with "wanna" and "gonna".) Maybe those are ok for a quick text message, but for other kinds of writing people should use the standard English forms.

Using "u" and other short forms is acceptable in chat and sms, where you have limits on your time and space. I agree with Goatish in that it really looks terrible outside of sms and chat. If you are writing in a formal/business sense, or even just writing to ask for someone's help, then writing "u" makes you look stupid and even rude (this also applies to wanna, gotta and their ilk). The general rule with writing is, if you have the time and space, then write it out properly.

When people are too lazy to write thw whole word "you" they write just 'u'. For u = for you Used usually in chat.

As others have said, 'u' is only acceptable in certain situations. I want to add that it's only acceptable in [i]private[/i] chat. You would never use 'sms language' in a conversation that could be read by anyone other than the specific person you're talking to, and you would only use it when typing is difficult (ie. on a phone). Never write 'u' if you are using a keyboard. People used to use '2' instead of 'to' and 'r' instead of 'are' in sms messages as well, but since touch-screens replaced the old keypads on phones I've honestly never seen those forms.