What is a Martingale Collar?
Martingale is a type of dog collar that provides more control over your dog without that choking effect of a slip collar.
These style of collars are extensively used in training and also helps keep your dog's head from slipping out of the collar during walks or when the dog gets too excited. A Martingale collar is made with two loops.
The larger loop is slipped onto the dogs neck and a leash is then clipped to the smaller loop. When the dog tries to pull, the tension on the leash pulls the small loop taut, which makes the large loop smaller and tighter on the neck, thus helping to prevent escape. 
When the collar is properly fitted, it will be comfortably loose without your dog noticing it much until the dog begins to pull.