If you don't even know what the B/C is (which I realized might be the case last night) it's referring to the forum for Breedable and Changing Pets.
Basically, someone draws art (either a base or unique art) and that art is sold or given away as someone's pet. There are lots of different shops with different themes and different requirements on for pets and their owners. Some require you to RP regularly, while others have no requirements, once the original price has been paid.
What makes B/C pets different from any other 'pet' art you can get in Minishops or wherever else you might find it is the Breedable part and the Changing part. Usually, pets will 'grow' through certain stages to reach adulthood or whatever equivalent. Once they get there, they can usually be 'bred' with another pet from the same shop and they have babies (which usually belong to the owners of the parents, unless the shop has other rules).