A debit card can also be ran through as a
credit, even though the money still comes from your
checking account. The difference mostly being when you run a debit, it asks for your
pin, and a credit will require you to
sign.
The only problems I can see with this are that anyone can sign your name, while hopefully only you know your pin number. Also, an electronic debit transaction is transfered immediately from your account to the account of whoever you're paying, whereas running a credit can take a couple of days, like writing a check. If you aren't careful with your math, this can screw you up, especially if you like to use the bank's 1-800 number to see your account at a glance, as they only reflect your account minus all recent electronic transactions. Check and credits still take a while to show.
And also, as I have noticed at my store, if you run a debit, and the money is not in there, the transaction will not go through. However, if you run it as a credit, it will go through regardless, and then you have a nice overdraft fee to worry about.
The moral of the story: Yes, that little plastic card is magical, but you've got to be careful.
-------------------------------------------
Ahem... due to numerous personal messages, I need to restate some of my information. The info I gave, it seems, was a little outdated.
XWiz informs me that in the UK, both debit and credit have had PINs for some time. Indeed he says that the magentic strip may not be around on said cards for much longer, at least in his corner of the world. SharQ has also told me, and I looked into it and it is truthful, that some debit transactions allow you to sign, and some credits require a PIN. I personally experimented to verify this.
I used my debit card to pay for a buffet at the Golden Corral, and made sure to tell them it was debit. They made me sign, but when I called the bank's number, it had gone through instantly, because it was a debit transaction. I then used my Mother's credit card (Mastercard) to make an ATM withdrawal. It required me to enter a PIN. I knew the number, because it's the same code Mom always uses.
That was all it took for me, XWiz and SharQ were correct. Here is an interesting aside, though. I did one last thing, using my debit card, I ordered a book online. It did not ask me to enter a PIN or an electronic signature, but identity theft is another node....