Dogs have assisted humans for centuries by being service dogs, therapy dogs, search and rescue dogs and filling many other roles, but one of the most important roles a dog can have is simply giving emotional support to the owner.
An Emotional Support Animal is a dog or other animal that assists a disabled or elderly owner at home and on airplanes by provided affection and emotional comfort. ESAs are not task trained and they do not have access to all of the places that service dogs do, but they help their disabled and elderly owners nonetheless. You must be considered disabled or elderly to qualify for an ESA. Having a diagnosis for a mental illness, by itself, is not sufficient unless that illness also disables you. Only a judge can truly determine that, but your doctor can probably tell you whether you are disabled or not unofficially, and they can prescribe an ESA. To qualify as disabled you must have limitations on one or more major life activity.
It is recommended that ESAs have decent obedience training. I would say a good measure for how well an ESA should be trained is the AKC's Canine Good Citizen test. This is not required, but strongly recommended. The test can be found here. If an ESA can pass the CGC, the ESA is decently trained. This is by no means decently trained for a service dog, but for a pet or ESA this will get the dog by.
The difference between ESAs and Psychiatric Service Dogs is often hard to follow. An ESA is a pet that provides comfort to the owner by just being there. A PSD is a task trained service dog that aids the handler by performing tasks. PSDs have public access while ESAs can only get into no pets housing and on airplanes. One must be disabled to have either a PSD or an ESA.
Editor's note: this article needs more material on the benefits of the human animal bond, including links to articles on the Delta Society site and medical research such as the papers of J K Odendaal. Volunteers willing to collect this information and compose a few paragraphs explaining the material to be added to this article are encouraged to contact the webmaster at http://www.servicedogcentral.org/content/contact