Overconfidence is what you've got when you're more sure of yourself than you should be. In a chess tournament, overconfidence might lead your opponent to underestimate you — which means you'll take her completely by surprise when you trounce her.
Confidence is a good quality. It means you are secure and believe in yourself. Overconfidence, on the other hand, means you feel certain about your abilities or eventual success in situations that are not so clear-cut. Your overconfidence might mean you fail your driving test, for example, because you're so sure you're a great driver that you don't practice parallel parking.