Any English-speaker who has ever tried to master French has had difficulty with the fact that, in French, there are TWO forms of the word "you" - "tu" and "vous" - and a whole set of complicated rules about which to use when (there are 2 actual verbs to describe which one you're using: "tutoyer" and "vouvoyer"). "Tu" is generally a familiar form (I "tu" my husband and step-kids) and "vous" is both formal AND plural (so if I were speaking to all of my step-kids at once I would say "vous").
But it gets way more complex than that, and represents for the non-native French speaker a social minefield. Do you Vous or Tu your boss? Well, it depends - probably Vous, but possibly Tu if you've known each other a while and you get along well. Your new in-laws? That also depends. I Tu my sisters-in-law but if my parents-in-law were still living I don't think I'd have the nerve, I'd probably Vous them. Someone older than you? Younger than you? Your neighbor? Depends, depends, depends. As with most things in French (and in France), there is the rule... and then there is the EXCEPTION to the rule.
Now, William Alexander has come up with a flowchart to help us figure this out.
I especially like the references to the 1968 Parisian riots and Woodstock. I was too young for either one of those events, but its nice to know you could be on a Tu basis with someone if you'd both been there.
At the end of the day, I try to follow the lead of the French person if I'm meeting him or her for the first time. If they Vous, I Vous. Period. Except that sometimes, I'm so used to Tu-ing in my home and family life that I occasionally slip up and Tu someone, say, at the post office.
A social faux pas, of course, and often I'll catch myself and apologize while correcting myself to Vous, but as I'm foreign and my accent will always give me away, they seem to overlook it.