A puppy is a baby dog. Unless you're more of a cat person, there's nothing cuter than puppies playing together.
When a dog is born, it's a puppy. Puppies, like most mammals, start out very small and quickly grow larger — in fact, some puppies' coats change color as they mature, resulting in a fully grown dog that looks completely different. In the fifteenth century, a puppy was specifically "a woman's small pet dog," from the Middle French poupée, "doll or toy."