For our typical cabinet box we use a combination of a blind dado joint and confirmat screws. We like the blind dado specifically because you get the overwhelming strength of a glued dado joint without seeing the joint from the exterior. We use the confirmat screws on any non-visible surface. This is an excellent piece of hardware as it not only provides lateral strength with the screw threads, it also provides immense sheer vertical strength due to the thickness of the shank.
A dado is simply a rectangular groove that is cut across the grain of a board. The joint formed by placing an intersecting board into that groove.
The dado joint allows the load on the board to rest along the full length of the dado, thereby giving it considerable load-bearing capacity. Because the end of the board is entirely encased by the dado's sides, the board can not cup or tilt. The dado joint does not offer any protection against the shelf pulling out of the side unless glued or fastened in some manner, and because the joint involves end grain, the gluing strength is limited.
Blind dados both begin and end shy of the edge. The benefit of these options is that the dado itself is not visible from one (half-blind/stopped, see below) or both (blind) sides. The board that fits into the dado must have one or both ends notched, and the notch should be oversized so the board has a bit of play when it is fitted in the dado to allow you to adjust the board flush with the case edge.
Stopped dados start at one edge but stop before they reach the other edge.
Dados that extend from edge to edge are referred to as through dados. These are the easiest form of dado to construct, however the dado is visible when viewed from the edge. This can be concealed by applying a face frame or other trim after the joint has been constructed.
Confirmat style connecting screws are designed for cabinets or other types of box construction. These unique fasteners act like a steel dowel forming a strong, stiff butt joint. They have a large shoulder under the head that locks the screw into the wood and prevents pulling through into softer materials.