In a framed cabinet, a number of construction techniques can be used to hold the box square and rigid. The choice of methods and materials used go a long way in determining the quality of the box.
The cabinet sides are usually dadoed into the face frame and rabbeted into the back. I-beams along the sides (or struts front and back) are best dadoed into the face frame, cabinet side, and cabinet back. In addition to or instead of I-beams, you may see corner blocks. Wood corner blocks are stronger than plastic or metal ones.
In box materials, look for 1/2" plywood sides and at least a 3/8" plywood back. The face frame should be made of solid wood.