Currently computer systems are available in many different sizes and processing abilities.  At the top of the spectrum of size and processing ability are the large mainframe systems.  These systems are typically quite large and have significant processing ability.  Beneath the mainframe systems in the hierarchy of computer systems are mini-computer systems.  The mini-computer systems constitute those systems that are smaller than the mainframes but still have significant processing ability.  Microcomputer systems are positioned one step lower in the hierarchy.  The microcomputer systems are often referred to as personal computers.  Lastly, the lap-top computers occupy the bottom rung of the hierarchy.  The lap-tops are relatively small in size and light in weight to facilitate portability.  The name lap-top implies that the systems may be used on the user's lap.