I actually just got done doing my first backyard comp, so let me try to explain.
The backyard competition typically consists of novice or amateur competitors. The competition itself typically consists of only cooking two meats, usually chicken and ribs, which means that you do not have to devote an entire night to cooking. In my competition I set up in the morning and the meat turn ins were done by 2:30 pm. The entry fee is typically lower for the backyard and there may or may not be cash prizes.
The pro category on the other hand is an all night and day competition, usually cooking 4 meats(brisket, pork, pork ribs, and chicken). Cash prizes are larger for the pros.
I hope that helps you out.