Though dogs and cats are different species that appeal to different types of people, they are similar animals. Cats and dogs are both mammals, but their ancestors diverged from the ancestral lineages millions of years ago. Cats and dogs are both predatory mammals that have relatively similar biology.
Cats and dogs have similar basic body plans. Both are four-legged animals with long tails and a horizontal body posture. Although most cats are smaller than most dogs, the two animals do overlap in size. As they both consume relatively similar prey, they both have similar digestive systems and teeth.
Both dogs and cats have fur to keep them warm, and they periodically clean their coats by licking them. Although humans have bred both in such a manner to produce dozens of color combinations, the wild ancestors of both animals were probably cloaked in muted earth tones to camouflage with their surroundings.
Cats and dogs were both domesticated by humans, although dogs were domesticated about twice as long ago as cats were. Both animals have served humans well and worked to earn their keep. Cats used to help control the rodent population for primitive people, while dogs helped protect humans from intruders and predators.