Are people who either enjoy or want to sing songs by other aritst to show off their talent
Most people who do the parodies are those who do Japanese songs, many people can be found on many websites that execute videos that people submit. Some are very good and some are very bad. Some parody singers are those who make other songs in other languages in the language they are most familiar with, (ex: japanese to english) they do that because they think that it would be easier to sing that way.
Some examples of parody songs can be found here:
Cantarella Dub in English