In English, "you", as a pronoun, is used in all types of relationships. However, in French there is a formal you - vous - and an informal you - tu. I was taught in school what the textbook difference was (i.e., your boss or parents versus your friends). I also know that it can be considered impolite if you use tu in the wrong situations. So I have generally always used vous when speaking with anyone in French.
Today I learned a new French verb: tutoyer (to use tu) which is used when one wishes to communicate on more familiar terms (i.e., switch from vous to tu). I learned it when one of my colleagues said I could tutoyer after I responded in the vous form in an instant message to him.
The funny part was when he told me that I could "tutoyer" him my brain thought that tutoyer was some sort of future tense of the verb "kill" ("tuer") so I thought he was telling me that I would kill him for something he did. When I told him that he replied: "mort de rire" - He was dying laughing!
...but at least now I could "tu" him!