The essence and key to success of expansion boards is standardization. By rigidly defining every dimension and contacts, expansion board standards allow you to plug almost any expansion board into almost any computer. But standardization is not a single issue. Expansion boards must be physically, electrically, and logically compatible with the motherboards with which they mate.
First of all, a board must fit into the computer for which it is designed. Consequently, today's expansion board standards dictate the physical size of expansion boards. In addition, the board must be able to send its signals to the motherboard and listen to those that the motherboard sends out. It must have some kind of electrical connection with the motherboard. Consequently, the physical standardization of expansion boards extends to their electrical connectors. Before signals can hope to get from one board to another, the connectors must mate together. They must match as to size, style, and placement so that they can properly fit together.
Although the physical size of an expansion board has no effect on its electrical compatibility, it is a major issue in compatibility. After all, an expansion board has to fit in the computer it is meant to expand. All expansion standards define the size of the circuit boards they use either as exact dimensions or as a set of maximum and, often, minimum dimensions.
Given his druthers—or even someone else's druthers—a circuit board designer would rather have more board space, acres upon acres of it stretching from here to the horizon to the limits of time and the universe. More space to work with makes the job easier. Unfortunately for the designer, practical concerns such as keeping the size of the computer manageable restrains his ambitions and board space.
On the other hand, smaller boards help manufacturers cut costs, at least after the price for developing new designs is paid. They require less in the way of the glass-epoxy base materials from which the boards are built. Consequently, setting the dimensions for an expansion board standard is always a compromise.
The prototype of all expansion boards was the card that fit into the original IBM computer of 1981. The bus and physical form of this board lurks behind all computer expansion to this day, evolving into Industry Standard Architecture and legacy expansion boards, now obsolete thanks to more modern designs.
The reigning standard is the physical size of the PCI bus standard, which specifies the slot and board size as well as other physical and electrical characteristics of expansion boards. Even though PCI electrically and logically differs markedly from the original legacy design, PCI still shows its heritage in several of its physical characteristics. The maximum board length and thickness both go back to the first days of the computer. The multipart basic design, too, harkens back to the first computer expansion cards. The same physical design also embraces the offshoots of PCI technology—PCI-X and PCI-Express.
Other expansion standards you're likely to encounter include the PC Card and CardBus standards, which from a physical perspective are identical (despite the electrical differences). Your desktop computer may also have a riser board for some of its features (such as sound and networking), which you may be able to upgrade or replace, much like an expansion board. Servers may use expansion boards based on the InfiniBand standard.