What is a mint mark on a penny?

A mint mark on any coin is a small letter or letters that indicate where a coin was struck. Not all coins have mint marks - for example all cents made in Philadelphia, and other coins struck there before 1980 do not have them, but other coins dated 1980 and later have a P mint mark. Other marks are S for San Francisco, D for Denver, W for West Point (on collectors' coins only); there were still other letters used in the 19th and early 20th centuries.

Other countries don't necessarily use mnemonic letters. Germany, for instance, uses letters that are assigned in the order that the mint was established. Berlin was the first mint, so it uses A, not B!