Open sided boards
The most common type of cornhole boards are Open Sided Boards. These boards have a plywood top, and the sides are framed with 2x4's or 1'4's.
The front edge of these boards is approximately 4" high, due to the height of the 2x4 plus the thickness of the plywood platform.
The rear edge is 12" high. Legs are often constructed that can be folded under the board after use. The ability to fold in the back legs makes open sided boards easier to haul than their box style counterparts.
2x4's are most commonly used to frame the sides of the boards. 2x4's are strong, and work well as the leg material, as well.
1x4's are sometimes used to create lighter boards. Usually a cross brace is placed underneath the boards to reduce bounce if 1x4's are used. Also, the rear legs generally require a crossbar of some sort for extra stability.