All of the different incarnations of Majin Buu as well as Bibidi and Babidi compose the Majin race in DBZ. This is not to be confused with the Majin Possession that Babidi uses to enslave people with. They are still their own race, just possessed by Babidi's magic.
The term "Majin" translated to mean "Evil Being" and is a general term that has been used in many different manga/anime series.
Related Threadshow to get majin - last post by @ Aug 23, 2009
Majin question. - last post by @ Sep 20, 2008
How to get the Majin? - last post by @ Mar 15, 2010
On Topic GD IV [Spoilers] - "I will destroy you!" Pretty hard to do when they've got Divine Majins - last post by @ Nov 4, 2010
Easy Majin - last post by @ Jul 17, 2009