When one learns a language one has to learn the culture as well...
One of the hardest things I've found when speaking French is to know when to call someone 'tu' and when to call someone 'vous'.
Vous is the formal/polite form and tu is the informal form. Vous is also used as a plural for addressing more than one person as 'you'. I've been told you use 'tu' for those people that you know well (friends, colleagues, etc), family members, your partner, children and animals. On the internet, however, especially if someone is around the same as as you, you use 'tu' almost immediately (well I do anyway and noone seemed offended by it!)
The act of 'tu'ing someone is called 'tutoyer' and the act of 'vous'-ing someone is called 'vousvoyer'. The title of this blog post means, "You can tutoyer me."
There are no hard and fast rules I've found as some people seem to call me 'tu' almost immediately yet some still call me 'vous' even though we've known each other for weeks, or months now! It's weird.
I remember borrowing French movie DVDs from the Alliance Française when I studied there and watching them, and noticing when they switched from the 'vous' form to the 'tu' form once the two strangers became friends. It's quite interesting!
Because English only has one word for 'you' (including the plural) I get in the habit of calling EVERYONE 'tu' which is kind of embarrassing and I often realise my mistake after I've said it, and quickly change to 'vous' which then changes the verb after (eg tu veux.. (you want to..) becomes vous voulez etc...) Oh là là !
I think it's one of those things I'll never 'get'!
(header image from here)