Brazilian Portuguese: What is the diference between “Tu” and “Você”?

If you learn the Brazilian Portuguese, you see that here, in Brazil, we use two words for “you”: “tu” e “você”. So, how to know which of these two to use? If I say “Tu”, all Brazilians will understand me? If I say “Você”, I'm being very informal?

Translate: Go you!
Photo by Cão Azul.

Calm down, I will explain everything! =)