KGLVR- The difference between pet and breeder snakes is a judgement call based on the physical quality of the specimin, and how well it conforms to a breed standard. It's the same as with purebred dogs.
Any snake is potentially capable of "breeding", but may not be a desireable "breeder". For example, lets say you have an Okeetee Corn snake. It's perfectly healthy, but has an overly dark wash in it's background color, and is small for it's age. This would be a great "pet quality" snake, but not something you'd want to breed. You're making this judgement based upon the accepted breed standard of what an Okeetee should look like.
I hope this clears it up for you...
BTW- I'm not an expert on breeding (yet), but Kathy Loves' book recommends a minimum size of 30" for breeders. I would imagine it's probably safer to be at least a few inches above that minimum. Especially with a female...