It is a truism that all hounds are dogs, but that not all dogs are hounds. What makes a hound, well, a hound? Is it genetic, ability, purpose, or all three?
A hound is a type of dog breed that was originally used for hunting due to its scenting (Beagle, Basset or Bloodhound) or sighting (Afghan Hound, Greyhound or Whippet) abilities.
To be more precise, here's the definition of the hound group from the AKC:
Most hounds share the common ancestral trait of being used for hunting. Some use acute scenting powers to follow a trail. Others demonstrate a phenomenal gift of stamina as they relentlessly run down quarry. Beyond this, however, generalizations about hounds are hard to come by, since the Group encompasses quite a diverse lot. There are Pharaoh Hounds, Norwegian Elkhounds, Afghans and Beagles, among others. Some hounds share the distinct ability to produce a unique sound known as baying.
Other national kennel clubs have a similar definition, according to Wikipedia.