A mint stamp is a stamp in its original issue state. It is unused, has never been mounted and it has all of its gum, if originally produced with gum.
Stamps are attached to envelopes or other wrapping (i.e., a package or box) that a customer wishes to send in the mail. The item is then processed by the postal system. A postmark, also known as a cancellation mark, is usually marked over the stamp and the wrapping. This marks the stamp as used and prevents its reuse.
Mint stamps are often more valuable than used stamps as in many cases fewer mint stamps survive. A mint stamp is usually in better condition than a used stamp, which has passed through the mail.
Sometimes, however, used stamps may be more valuable than mint ones if high numbers of mint stamps have survived. This may be because large numbers of mint stamps were bought by collectors but only a few were used on letters.