When do I use "tu" and when do I use "vous"?

Explanation:

Both "tu" and "vous" mean "you" but "tu" is used for the singular of "you" and "vous" is used in two cases: the plural of "you" and for the singular but formal version of "you".

Example:

TU:

"Tu es intelligente." (You are intelligent --> Singular/Informal)

VOUS:

1) "Vous etes intelligents." (You are intelligent --> Plural, note there is so formal plural way of saying "you")

2) "Vous etes intelligente" (You are intelligent --> Singular/Formal)