The Rowdyruff Boys are the villainous male counterparts to the Powerpuff Girls. The trio consists of Brick, Boomer and Butch.
Mojo Jojo created the Rowdyruff Boys by flushing armpit hair, snails from the cafeteria, and the tail of the talking dog down a jail cell toilet that contained the apparent potency of Chemical X ("snips, snails, and puppy-dog tails"). They have very similar abilities to the Powerpuff Girls, but they all possess very easily targeted weaknesses, originally an aversion to Cooties (fixed by HIM), shrinkage when their egos are damaged, and an obsession with destruction that can lead to their own downfall.
In a sense they can be seen as a dark reflection of the Powerpuff Girls, seeing as how Mojo Jojo had a hand in the creation of the Girls as well as the Boys. The key difference being that while the Powerpuff Girls were created by accident when Mojo Jojo (still Jojo) shoved Professor Utonium, accidentally breaking the bottle of Chemical X, the Rowdyruff Boys were created by Mojo on purpose. Mojo also has the opposite feelings for the Rowdyruff Boys in that he sees them as his children, while he outright despises the Powerpuff Girls.