A credit card uses money you don't actually have in an account. You have an available "credit line", which is a certain amount you can spend. Then every month, you have to pay your bill of the money you spent.
A debit card, however, uses money you have in an existing checking account. It's like the credit card form of a check. You can't use money you don't have in that account, and if you do, you get charged fees.
I hope this makes sense.