Tu is the familiar form of the word ‘you’, usually used with friends and family. Vous is the polite form, used with people you don’t know well, or people who are in a position of seniority. General rule: use vous with the neighbouhood baker and your boss. But the under 40 crowd tends to be more informal. Don’t be outraged if tu is commonplace in your office. When in doubt, let the native decide.