The two forms, "u" and "you" may have the same meanings but otherwise they have very different registers, and usages. Learners should avoid writing; "u", "b4", or "2nite", etc. in any formal correspondence if they wish to create a positive impression in their readers. In texts, IMs (instant messaging) and emails to friends these abbreviations, also known as textese, txt-speak, and texting language are more than acceptable and can save the writer time and space.

Yes, it's the same, just like when people use "r" instead of "are," though I would stray away doing that.