A collar is a training tools to use to train your dog. No collar is cruel, however how the owner chooses to use the collar can be cruel.

Choke Chains used to be the most common collar used, and now is uncommon to see. It is a slip collar that has no stopping point when tension is applied, causing pain and the skin/hair to pinch. Many a dog have died from wearing such a collar and catching it on a fence, doorknob or other common object. 

This is a nylon slip collar. Slip collars come in chain, leather, nylon, and cotton. All slip collars have the same risk for pain, pinching, and the possibility of your dog catching it on a common object and hurting itself or causing death. Slip collars can be useful is training during certain situations, however should be removed after training.

A prong collar is a type of limited slip collar. Trainers often refer to this type of collar as "power steering for dogs" as it is useful when training a leash reactive or strong pulling dog that weighs more than the trainer/owner. If used correctly, a prong collar is more humane than a choke chain. The prongs apply even pressure around the dog's neck. Too much force can cause the skin to pinch between the prongs, especially if fitted improperly.