Due perhaps to some Victorian notions about progress, warm-blooded animals were considered “higher” and more advanced than cold-blooded ones, which were grouped together on the low end of the phyogenetic tree. Thus birds (of mysterious origin) and mammals (coming from mammal-like reptiles) were considered close. The study of everything cold-blooded but with a skeleton (coming from fish, not an arthropod or mollusc) was herpetology. The organisms thus lumped together are called “herps” for short. But gradually we realized that turtles, snakes & lizards, and frogs aren’t really very similar.
PZ Myers on Pharyngula supplied the diagram that contrasts our traditional divisions of culturally “similar” animals with their real descent. He also throws in a cartoon about herpetologists.
Birds are descended from active, predatory, warm-blooded dinosaurs, which are descended from early reptiles, as are crocodiles, turtles, and lizards & snakes. Reptiles and their descendents except for birds is a paraphyletic group because it excludes some descendents of the common ancestor. Reptiles and all their descendents is a proper monophyletic group. Lizards & snakes, Frogs, and turtles are a polyphyletic group, because they exclude groups that are closer to the included groups than those groups are to each other. Birds plus mammals is also polyphyletic.
I thought mammals came from mammal-like reptiles, but we now know that mammals and reptiles are sister groups with a common ancestor. (See Amniota.)