Open Question: Please explain what it means to have an overdraft,and I have a credit card Q too...?

I went to the RBS today to ask about an overdraft,lol but I didnt understand. Could somebody please explain it to me. Is it similiar to a credit card? And it means you can't take out that money,you just use that card? And how do you pay them back if you use that overdraft money? Overdraft for £500 for example.