Digital or analog
A fundamental decision in designing a computer is the choice of whether he will be digital or analog system. When digital computers are working with discrete numerical or character variables, the analog designed to handle continuous flows of incoming data. Today, digital computers have a much wider range of applications, although their analog counterparts are still used for some special purpose. It should also be mentioned that there might be other approaches, for example, in pulse and quantum computing, but so far they are either very specialized or experimental solutions.
Examples of analog computers, from simple to complex, are: the slide rule, an astrolabe, an oscilloscope, television, analog audio processor, the autopilot, the brain.
Among the most common discrete computers known abacus, or ordinary bills, the most complex of such systems is a supercomputer.