In English, the second person subject pronoun is always "you," no matter how many people you're talking to, and regardless of whether you know them. But French has two different words for "you": tu (listen) and vous (listen).
The difference in meaning between these two words is very important* - you must understand when and why to use each of them. Otherwise, you may inadvertently insult someone by using the wrong "you."
Tu is the familiar "you," which demonstrates a certain closeness and informality. Use tu when speaking to one
- peer / colleague
Vous is the formal "you." It is used to show respect or maintain a certain distance or formality with someone. Use vous when speaking to
- someone you don't know well
- an older person
- an authority figure
- anyone to whom you wish to show respect
- familiar and singular: tu
- familiar and plural: vous
- formal and singular: vous
- formal and plural: vous
Because the tu / vous distinction doesn't exist in English, beginning French students often have trouble with it. Some people follow the guideline of using whatever the other person uses with them. This can be misleading: someone in authority may use tu with you, but that certainly doesn't mean that you can respond in kind. You can try asking On peut se tutoyer ?, but when in doubt, I tend to use vous. I'd rather show someone too much respect than not enough!
*There are even verbs to indicate which pronoun you're using:
tutoyer = to use tu
vouvoyer = to use vous
Quiz on tu vs vous