Credit: Every time you use a credit card, you are actually borrowing money that is made available to you by a bank or other financial institution. The institution pays the debt to the vendor, and in turn, you pay the money back to the institution. By signing up for a credit card, you agree to pay back the money that you borrowed, in addition to any interest drawn on the amount you borrowed.
Debit: Issued by your bank or other financial institution, debit cards are check cards that take funds directly from the money that you have in your bank account--in a sense acting like a check, just faster. With a debit card, you don't have to carry cash or checks, and it is very convenient to shop at a variety of places including gas stations, grocery stores, restaurants, and retail stores. They provide instant access to your money and are accepted worldwide.
Debit cards are also used like credit cards (for places that do not have the debit option). Meaning that the store you are shopping at 'swipes' it and then you sign off on the receipt. You don't have to show a picture ID, and there is usually no PIN# for you to punch in.
Using it as a debit card however, you have a PIN# to enter but do not need to sign off on a receipt.
So:
Credit Cards are for money that's borrowed & you're paying back (a loan).
Debit Cards are for money that you already have.