"Thanks" is more of an informal way of saying it whereas "Thank you" would be more formal. You can also think of it as "Thanks" is "slang", and can be used with family and friends and "Thank you" should be used with all other people.

If you are in a formal situation, or don't know the person well, or the other person is above you in age or status, say "Thank you". or "Thank you, sir". or "Thank you very much, I appreciate you". If it is a casual situation, or your are comfortable with the other person, or he is your friend, you can say "thanks". If you are being sarcastic, or really mean the opposite, you can jokingly say "Thanks a lot...for nothing". Or "Thanks a lot!"

Hello! I think "thanks" is more informal then "thank you". Most people say "thanks" when they're speaking with people they know, people which they have an intimate relation. For example, friends and family.

"Thank you" is more polite, more elegant than "Thanks". Note: that thanks you isn't possible. Thank you very much. OR Thanks very much. (NOT Thanks you.)