You. A fine example both of Brazilian Portuguese language evolution and the attenuation of treatment formality in our society.

No, not you actually -- what I was gauchely trying to mean is that the Portuguese pronoun “você” is almost completely equivalent to the English "you", if not semantically, at least grammatically.

Você is a pronoun for referring to the second person (the one to whom the speaker speaks) but the verbal forms associated with it are the same associated to the third person (about whom the speaker speaks).

For example, if you need to say "You wrote this to me" in Brazilian Portuguese, you should say "Você escreveu isso para mim". Você takes the place of the modern English you, the archaic thou or the Portuguese "tu". But escreveu is the third person form, (past tense, in this case) of the verb escrever, which means to write - and it’s the same form used in Ele|Ela escreveu.

If you feel confused by the talk about "first person", "verbal form", etc., here's a little table that should help you to figure out the inherently messy structure of the Portuguese language verb usage. The table presents all possible variations (regarding to who executes the action implied by the verb) of the present tense form of the verb escrever.

                                Pronoun:        Variation
                                --------        -----------
                                Eu              escrevo
interchangeable with 'você' --> Tu              escreves       
                                Ele             escreve   <-- used with 'você'. 

                                Nós             escrevemos
interchangeable with 'vocês'--> Vós             escreveis
                                Eles            escrevem  <-- used with 'vocês'.

Now you may feel inclined to ask, as I was: why did we, the speakers of this mild tongue, cross the second person pronoun with a third person verbal form? The answer is that no one have crossed anything, at least not in the way it first seems. This is because 'você', however being used in place of a personal pronoun, is in fact a treatment pronoun, just like 'Your Eminence, 'Your Grace', 'Your Excellence', etc., and treatment pronouns are traditionally associated with the third person. This is so because it's supposed to be a disrespect to refer directly to the “self” of someone important, so we refer to them by their title.

The word "você" is a four-step reduction of the formal treatment pronoun "Vossa Mercê", which was largely used to refer to someone of greater importance without regarding his/hers specific title, if any. As the years passed and the usage became more and more widespread, “Vossa Mercê” became “vossamecê”, and “vossamecê” became “vossmecê” (largely used until the middle-twentieth century) and then finally simply “você”. In one of those semantic twists, typical to the evolution of languages, today the pronoun “você” is no longer used to refer to someone important or deserving special respect, but otherwise to people with which there’s no need of formality, like family members, friends and in almost any non business related situation.

To sum it up: in Portugal you should use 'tu' when talking to friends and to people of equal or lower "social status" (whatever you think that means) and 'o senhor' (masc.) or 'a senhora' (fem.) when talking to strangers or people to whom you must pay respect, like authorities and older people in general, and você when talking to strangers not older than you.

While in Brazil, you can use either 'você' or 'tu', depending on the region you are on at the moment. In the states of Rio Grande do Sul, Santa Catarina, and Rio de Janeiro and all over the Northeast*, it’s very common to use 'tu' instead of 'você' when talking informally – in any other region 'você' is predominant. 'O senhor' [m.] and 'A senhora' [f.] is ubiquitous in the whole country for formal situations.

Think it’s chaotic? Well, wait until you need to learn accentuation rules...

At request of the audience, I should note that the Spanish 'usted' is of analogous origin: the ancient "Vuestra Merced" went through a process of shrinking until it reached its modern form. The Spanish language, however, kept the formality bearing of this pronoun.

* This is because pure Portuguese descendants were predominant in these regions until the end of the ninetieth century.