Source: Despicable Me franchise (2010) Type: Fan-made
Minions are yellow henchmen who have existed since the beginning of time, evolving from yellow single-celled organisms into beings who have only one purpose: to serve history's most ambitious villains. Thus they are succinctly described on Wikipedia. And they'll be the stars of their own film (prequel spin-off of Despicable Me) in 2014. I do have a question about how the little guys became an internet meme though (Celebrity Minions). I mean, if they're built to be henchpersons, why imagine them as a) heroes and b) stars?
The answer is probably in their simple but pleasant graphical look. Great for costume parties where you only want to make up your thumb too.