A "hound" is a dog from one of several breeds often trained to hunt. This term originated in old English when fox hunting was a popular sport for royals and nobles. Hunting dogs were known as foxhounds. A hound typically has a long face and droopy ears.
Hounds track and hunt a variety of game including foxes, rabbits and ducks. Hound is often used in casual conversation in lieu of "mutt" to refer to any dog. The term has also evolved to refer to a person who aggressively pursues something. One might say: "Jim hounds me all the time to give him my concert tickets."