Giant pandas are peculiar animals. While they look like bears, they aren't quite bears; and while they're built like raccoons, they aren't quite raccoons, either. We know them by their cuddly depictions and black ears, eye patches, arms, legs and tails. Taxonomists know them for having an especially puzzling biological history.
The giant panda has been around for more than 600,000 years, occupying far broader areas of China than it does now. While pandas are herbivores, they are descended from the carnivorous ancestors shared by raccoons, bears, cats, and dogs.
Despite their short digestive systems and their resultant difficulty absorbing nutrients from plant matter (requiring them to eat almost constantly), pandas, or at least their close ancestors, gave up meat at some point in their history and came to subsist almost entirely on bamboo. The lack of mobility caused by the giant panda's need to spend so much time eating can be blamed, partially, for how steeply the animal's numbers have declined in response to human expansion.
The first Europeans to take note of the giant panda called it a species of bear. After all, it looks just like one. Later, scientists studying panda skeletons found them to have more in common with the smaller, and appropriately named, red panda - a close relative of the raccoon. Thus, it was considered to be part of the raccoon family for a while.
Taxonomists have recently placed the giant panda back in the 'bear' category, although it does differ significantly from bears. The giant panda, unlike its bear cousins, does not hibernate seasonally, tends to avoid eating meat, and has well-developed molars for crushing plant matter.
It also has an extra sixth digit on each of its front paws (used for holding bamboo), an extension of the wrist bone which acts quite like a thumb. The red panda has a thumb like this too, although in the red panda's case, the thumb is not quite so well-developed. And, of course, one can't forget the giant panda's ultra-raccoon-like eye patches.
It might be best to allow the giant panda a category of its own. While it shares characteristics of raccoons and bears, it's hard to fit it into either category, especially since it's a vegetarian species. Until a better answer is found, the iconic giant panda will likely be forced remain a taxonomic hot potato, tossed back-and-forth between the bear (Ursidae) and raccoon (Procyonidae) families.