I was reading this first-hand account of Pet Battles in Mists of Pandaria and found an interesting question the author wasn't sure about.
On the way back to Audrey, I caught a Black Lamb, a Squirrel, and finally the loathsome Stormwind Rat. The Squirrel is a rare-quality pet and the Stormwind Rat is a poor-quality pet, but I wasn't entirely sure what that meant.
What is the difference between different quality pets?