Every time you use your card the merchant has to pay a fee for the ability to accept that type of payment. Usually the fee is anywhere from 1.5%-3% of the sale plus a small transaction charge. Truecostofcredit.com is a site that lets you see just how much your credit card charges retailers every time you use it.
Why would you care to know? Well, to make up for that loss on every credit card sale merchants have to charge more somewhere else...