Learn French Grammar: When do I use tu and when do I use vous?

The words tu and vous both mean you, but the choice between the two words in French depends on the person being addressed.

 As a general rule, tu is always used to address a single person, but it's used also to adress somebody in similar social situation, similar social situation means things like:

Your role in the current "speech context"
Job status (junior vs. boss)
Where the person you're speaking to fits in your "social network"
Your attitude towards the person you're speaking to.

 So the choice of tu or vous has to do with age, but also to do with other things.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

 Use tu when speaking to one:
        peer / colleague
       Here are some more concrete examples:

Pupil to teacher (vous):This is an analoguous situation to the 'hierarchy gap' mentioned above.
Young child to adult (vous): A young child talking to an adult.
Teacher to primary school pupil(tu):"tu" is generally used for talking to young children in any social situation.