A debit card is a bit like a cheque book.
It is tied to one bank account, and if you want to pay for something you hand over your debit card and the money is taken straight out of your account.
If you do not have enough money in your account to pay for the item you cannot buy it.
(the main difference with a cheque book is that you CAN write a cheque out even if you have no money in your acount, but it will bounce).
A CREDIT card is not tied to a bank acount and you can use it even if you have no money, so it is a sort of short term loan.
When you buy something with a CREDIT card the company who own the credit card (such as Visa) actually pay the shop (after taking their discount).
The CREDIT card company then send you a monthly bill showing all you have bought on the Visa card, and you then pay your money to Visa to pay off the "loan".
If you dont pay it all off they charge you interest on the "loan".