Embedded computers are meant to do a single or a few very specific tasks. However, the tasks are not always extremely simple. Multiple embedded computers can be a very cost effective way of accomplishing a complex task, by breaking down the steps into very discrete units. This can be seen as similar to the function of a CPU, which executes programs through very basic steps. However, general purpose CPUs can execute a variety of different programs, while the embedded computer is executing a single function.
Netrino.com, which call themselves the embedded system experts, provide a quote which points out both the ubiquity and importance of embedded computers:
Of the nine billion processors manufactured in 2005, less than 2% became the brains of new PCs, Macs, and Unix workstations. The other 8.8 billion went into embedded systems. The essence of every modern electronic device, from toys to traffic lights to nuclear power plant controllers, these processors help run factories, manage weapon systems, and enable the worldwide flow of information, products, and people.
Embedded processors span the range from simple 4-bit microcontrollers like those at the heart of a greeting card or children's toy, to powerful custom 128-bit microprocessors and specialized DSPs and network processors.
Embedded computers can be very simple and in a portable device, very simple and in an immobile device, or have many discrete embedded computers in either portable or stationary devices. For an example of a large portable device; there may be numerous embedded computers in a car, doing a single task, or measuring a specific value. They have no graphical display, and generally are unable to be programmed, but your auto mechanic has a reader which can collect the output from the embedded computers in your vehicle and use the information to diagnose problems with the car. Nuclear Reactors are large and (we hope) stationary devices, and may contain many embedded computers which perform single tasks or evaluate specific measurements of how the reactor is functioning.
Your washing machine may contain several embedded computers, and nowadays repair people remove non functioning embedded units and replace them with a functioning unit to fix many problems, rather than testing various wires running from motors to other parts of the washing machine. Embedded computers even tell the repair technician where the problem unit is.
Your digital watch is another example of a computer- a very limited one with one or a few embedded computers on chips, depending on how many functions your watch has.