Another controversial item in the trainer's kit. In my opinion it is important to educate yourself on all the training tools and make informed choices about what you are comfortable with and what suits your dog. An electric collar can be a very useful tool, it can also be used to completely ruin a dog's training (as can most training tools!). At it's most useful, it can help with distance commands such as positions and recalls, and it removes the hands-on element of correction which can help with sensitive dogs. It must be fitted snuggly and high on the neck like the prong, and the hair should not prevent good contact with the dogs' neck. Different collars operate differently and have different stimulation so there can be some experimentation to figure out the right brand for each dog. Some collars increase drive or excitement and others suppress it.
As far as when and how to use an electric collar, ideally an experienced trainer should be consulted. They can show you how to introduce a dog to the collar, the difference between using the collar for negative reinforcement and punishment and where those methods may overlap. We typically introduce the collar around 6 months of age, and then begin using it when the dog is showing a high level of independence. Our first exercises include negative reinforcement and teaching the dog to "switch the collar off", then we move to recall training, for us a reliable recall is essential because we enjoy hiking our dogs off leash.
Another item in this category is the electric anti-bark collar, this can be useful when traveling and at trials, typically barking dogs are not well tolerated by surrounding neigbors.