A shotgun house was very common in the days before air conditioning. If you weren't lucky enough to have some big trees to shade your house, one of the best things you could do was build it in this shotgun style.
This basically means that half the house is on one side of an open breezeway, and half is on the other. This helps air circulate around the rooms and keeps it a bit cooler than it otherwise would be. The name comes from the fact that you could sit on the front porch, shoot a shotgun through the house and hit something in the back yard.
The amazing thing is that people used to get along just fine without air conditioning. My grandmother wouldn't have one for years, even after they were available and fashionable. She said it made the air seem funny.