Box joints are the "poor man's dovetail joint". It has relatively good mechanical strength, but does not have the interlocking capabilities given by the sloping of the dovetail pins and tails. The box joint does still provide an incredible amount of gluing surface, making it a strong joint.
The basic box joint is formed by creating a regular series of slots and fingers, the length of which is the same as the stock's thickness. It is recommended that the width of the fingers also be the same as the stock's thickness since this gives the optimum strength for the least amount of cutting. Having thicker fingers actually reduces the gluing surface, weakening the joint. Having narrower fingers, however, can dramatically increase the gluing surface, at the cost of more labour to construct the joint.
In terms of aesthetic appeal, the box joint does suffer somewhat by its patterened appearance, but this can be overcome by some creative alternatives.
The standard box joint has all its fingers and slots the same width. The joint can be given some visual excitement by varying the width and spacing of the fingers that make up the joint. This does increase the difficulty in cutting the joint, and measurements must be carefuly made because the joining boards will no longer have identical setups.
If the joint being formed does not conform to a 90º angle, an angled box joint can be created. This is accomplished by placing the board on the required angle while passing it over the tablesaw/router while creating the slots. Just remember to keep the same side "forward" as you pass it over the cutter in order to create a matching angle.
When you want the box joint to be visible from only one side, you can take advantage of the half-blind box joint. This is formed the same way as the half-blind dovetail, but is considerably easier to cut, and it can even be done on a tablesaw.