A regular collar (or buckle collar) is adjustable and buckles around the dog's neck.  If you get it comfortably snug, these collars work fine-most of the time.  But how many times has your dog pulled back and slipped right out of his collar?  I would guess more than once.  Enter the martingale collar.  These collars were first designed for sighthounds, such as greyhounds, salukis, afghans, and so on.  The sighthound typically has a smaller head than neck.  That makes it quite difficult to make an effective collar when a dog begins to pull back.  First you adjust the collar considerably looser than a regular collar.  The dog starts off in a more comfortable position.  But as he begins to pull, forward or backward, the collar tightens up in a humane manner and keeps the collar from slipping off.  These collars are typically marketed primarily to greyhound owners, but work much better than a regular collar on ALL types of dogs.  We have put these collars on everything from a lab to a shi tzu, and they just make for a safer collar and a happier hound.