This is a very important difference between French and English.
English no longer distinguishes between the singular and the plural formal version of “you”, in French it is important to know when to use tu and when to use vous.
Vous is the plural form of you. This is somewhat equivalent to you all, you guys or all of you (except that it does not carry any familiarity when used with the plural). For example you can use it to adress your friends as well as talking to a group of colleagues or unknown people :
Que faites-vous ? 
What are you (all) doing ?
Vous is also used to refer to single individuals to be polite, to show respect or to be neutral. It is used when talking to someone who is older than you, to your boss or to someone with whom you are unfamiliar. This is called le vouvoiement
Tu is the singular and informal form of you. It is commonly used when referring to a friend, to a family member, to a kid or to someone much younger than you. This is called le tutoiement . If it is used when speaking to a stranger, it shows disrespect. As a rule of thumb, use tu only when you would call the person by his first name, otherwise use vous.
Maybe it seems a bit complicated but don’t worry French people will usually make it known when they would like you to refer them by tu.
Examples :
| Informal
Tu vas bien ? Comment t’appelles-tu ? Tu es d’où ? Je peux t’aider ? Quel âge as-tu ? |
Formal
Vous allez-bien ? Comment vous appelez-vous ? Vous êtes d’où ? Je peux vous aider ? Quel âge avez-vous ? |
English
How are you ? What’s your name ? Where are you from ? May I help you ? How old are you ? |