A guide dog is a dog individually trained to assist a person who is blind with navigation. They are trained to recognize and navigate around common obstacles such as pot holes, utility poles, curb cuts, mail boxes, and low-hanging branches, among other things.
There are several programs that train dogs to guide the blind. The most well known of these is the Seeing Eye, in Morristown, New Jersey. Only dogs that graduate from the Seeing Eye are properly called "Seeing Eye Dogs," all other dog guides are called "guide dogs."
In some countries the term "guide dog" is also used to refer to assistance animals of all kinds, but in the U.S., the term "guide dog" refers specifically to dogs who assist owners who are legally blind.
Most people who are blind are not totally without vision. Generally, a person is considered blind when their vision is below 20/200. In order to succeed with a guide dog, the person must have too little vision to allow them to try to guide the dog as this would damage the dog's training.
Guide dogs assist people who are blind. They indicate to their blind handler when an obstacle is in their path, usually by stopping and standing still. On command, they will attempt to find a way to navigate around the obstacle.
They can also be trained to find specific places and things on command. Guide dogs generally learn the places where the handler goes regularly, such as home or work. They can also find things like chairs, restrooms, and exits on command.