Remember that copy of Duckhunt at the old video store that was a little different from other copies? What about that copy of Super Mario Bros. 4 at the rental place. Well, both of those games are pirates. Pirated games and unlicensed games are often confused, but an easy way to tell them apart is to remember, unlicensed games are produced, pirate games are altered. Most pirated games are produced by hacking the code of another game. Generally the hackers would change the sprites and a few other details in an attempt to pass of their creation as a new game. Pirated games range in quality based upon the quality of the source and the skill of the hackers. 

The NES was one of the most pirated consoles in history. This is probably due to its position as the king of all systems in the late 1980's. As far as I know games are still being hacked for the NES to this day. Just take a look at some of the more recent titles on the list below.