- TU [You]: Used to talk to your friends for example or in informal conversations. It's a personal pronoun.
- VOCÊ [You]: Used in more formal situations, it's a treatment pronoun. You can use when talk to older people or to officials for example.
This is the "official" concept of them. You can see this clearly in Portugal, Angola or the other countries where people speak the European Portuguese.
In Brazil is a different case. In brazil, the VOCÊ is used as personal pronoun in some regions and they never use TU, they use VOCÊ when talk to anybody and even in informal situations. (Ex.: São Paulo, Curitiba)
In other regions of Brazil (actually majority of them) people use TU for both situations, formal and informal. (Ex.: Rio de Janeiro, Recife, Porto Alegre)
Is interesting that people use to say that in Brazil people use "almost always" VOCÊ, when the oposite situation actually happens. Anyway in all regions in Brazil, people use VOCÊ with the same "meaning" as TU.