Well... in classic portuguese, "tu" is the informal and "você" the formal one. "Você" is a word that suffered a lot of mutations to take this form, originally being "vossa mercê", a very formal way to call someone. That's also the explanation of being used in the "third person". Here in Brazil both are used as informal. We use "o senhor" or "a senhora" as a formal way to call someone. "Tu" is most used in the southern states and "você" in the rest of the country. And normally "tu" is conjugated using the "third person" like junior explained. But in Portugual they still uses "tu" as a more informal way.

well, actually ''tu'' is more often said in Southern Brazil, that's where I live... but you can say both ''tu'' and ''você''... but ''tu'' is much more informal than ''você'', so there are moments that you can't say ''tu'' for example, when talking to a boss or an older person... in informal speech, don't worry about conjugating ''tu'' correctly as grammar books say, because we conjugate ''tu'' the same way as ''você'' for example: você foi >> tu foi (you went) você quer >> tu quer (you want)

really,"tu" is informal and "você" is formal, but you can use both of them, but to old people use "senhor(a)"

'cause the same way that Usted is more formal in spanish, Tu is more formal in portuguese. In Southern Brazil they used it more than another parts . In São Paulo for example, we use Você = Tu , but the verbal conjugation is different so.

Everybody gave a little explanation and I should agree with then, so i should say that our portuguese came from the portugal portuguese and there they use this kind of structure, here we had mixed that original portuguese with a lot of others languages so whe do not use a lot of old portugtuese, it's correct to use TU, and if you do correctly it's not informal as people say, it's because people do not know how to use this correctly, "Tu foste me procurar?" "Você foi me procurar? " " was you lokking for me?" so the firt one is absolutly formal, and the second one is normal, but if you do not conjugate the verb in the right way "TU foi me procurar?" it becames really informal, and a quite bit bad on listening seens that you do not know a lot of portuguese or you are a diferent style of spech. in a short answer we do not use TU, because você is easyer! and everybody uses. :P