What is the difference between "tu" and "vous"?

"tu" is a far more relaxed and friendly way of saying "you" in French. It can be used for friends and family. Don't use it with an adult or someone you don't know unless they have told you that you can - it is seen as rather rude and disrespectful.
"vous" is for formal situations, people you don't know and for situations when you should be showing respect. Using it shows that you are treating the situation formally and "vous" is the go to "you" in any conversation outside close family and friends. If in doubt, use "vous" and wait to be told otherwise.