A computer module is a selection of independent electronic circuits packaged onto a circuit board to provide a basic function within a computer. An example might be an inverter or flip-flop, which would require two or more transistors and a small number of additional supporting devices. Modules would be inserted into a chassis and then wired together to produce a larger logic unit, like an adder.
Modules were the basic building block of most early computer designs, until they started being replaced by integrated circuits in the 1960s, which were essentially an entire module packaged onto a single computer chip. Modules with discrete components continued to be used in specialist roles into the 1970s, notably high-speed designs like the CDC 8600, but advances in chip design led to the disappearance of the discrete-component module in the 1970s.
In the 21st Century, computer modules more commonly refer to computer-on-module used in embedded computing. An extension of the concept of System on Chip (SoC), COM lies between a full-up computer and a microcontroller in nature. Today's Computer On Modules (COM) are complete embedded computers built on a single circuit board. The design is centered on a microprocessor with RAM, input/output controllers and all other features needed to be a functional computer on the one board. However, unlike a single-board computer, the COM will usually lack the standard connectors for any input/output peripherals to be attached directly to the board.
The module will usually need to be mounted on a carrier board (or "baseboard") which breaks the bus out to standard peripheral connectors. Some COMs also include peripheral connectors and/or can be used without a carrier.
A COM solution offers a dense package computer system for use in small or specialized applications requiring low power consumption or small physical size as is needed in embedded systems. As a COM is very compact and highly integrated, even complex CPUs, including multi-core technology, can be realized on a COM.
Using a carrier board is a benefit in many cases, as it can implement special I/O interfaces, memory devices, connectors or form factors. Separating the design of the carrier board and COM makes design concepts more modular, if needed. A carrier tailored to a special application may involve high design overhead by itself. If the actual processor and main I/O controllers are located on a COM, it is much easier, for example, to upgrade a CPU component to the next generation, without having to redesign a very specialized carrier as well. This can save costs and shorten development times. On the other hand, this only works if the board-to-board connection between the COM and its carrier remains compatible between upgrades.
Benefits of Computer Modules 
There are many benefits to using COM products instead of ground-up development. These benefits include increasing speed to market, reduction to risk, cost savings, choice of a variety of CPUs, reduced requirements and time for customer design, and an ability to conduct both hardware and software development at once.