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

Tu and vous both mean 'you', however tu is much more informal and familiar and vous is more formal and polite. You would say tu when speaking to a friend, family member etc, and vous when speaking to an adult you don't know, a teacher or in a formal situation.
tutoyer means to call someone tu
vouvoyer means to call someone vous
It is often asked when first meeting someone: On peut se tutoyer? (Can we call eachother tu ?)