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When most people think about computers, they picture a personal
computer, or PC. This type of computer is called personal because it is
designed for only one person to use at a time. Personal computers fall
into several categories that are differentiated from one another by their
sizes. The most common sizes are:
◾ Desktop PC: A computer
designed to be
used at a desk, and
seldom moved. This type
of computer consists of a
large metal box called a
system unit that contains
most of the essential
components, with a
separate monitor, keyboard,
and mouse that all
plug into the system unit
(see Figure 1.2).
◾ Notebook PC: A portable
computer designed
to fold up like a notebook
for carrying, as
shown in Figure 1.3.
The cover opens up to
reveal a built-in screen,
keyboard, and pointing
device, which substitutes
for a mouse. This type of
computer is sometimes
called a laptop. A smaller
version of a notebook
PC is sometimes referred
to as a netbook (which
is short for Internet book,
implying that this type of
computer is primarily for accessing the Internet rather than running
applications).
◾ Tablet PC: A portable
computer that consists of
a touch-sensitive display
screen mounted on a
tablet-size plastic frame
with a small computer
inside, as in Figure 1.4.
There is no built-in
keyboard or pointing
device; a software based
keyboard pops up
onscreen when needed,
and your finger sliding
on the screen serves as a
pointing device.

Multi-User Computers
Multi-user computers are designed to serve groups of people, from a
small office to a huge international enterprise. Here are some common
types of multi-user computers:
◾ Server: A computer dedicated to serving and supporting a network,
a group of network users, and/or their information needs. Many
networks employ servers to provide a centrally accessible storage
space for data, and share common devices like printers and scanners.
A small network server may look similar to a desktop PC, but may
have a different operating system, such as Windows Server or
Linux. A large server that manages a wide-ranging network may
look similar to a mainframe. A group of
servers located together in a single room
or facility is called a server farm, or server
cluster. Large Internet service provider (ISP)
companies maintain extensive server farms.
◾ Mainframe: A large and powerful computer
capable of processing and storing large
amounts of business data. For example, a
mainframe might collect all the sales data
from hundreds of cash registers in a large
department store and make it available to
executives. The modern mainframe unit itself
is a large cabinet, or a series of cabinets,
each about the size of a refrigerator (see
Figure 1.6). A mainframe may be stored in
its own air-conditioned room in a business
or school, and may have multiple employees
monitoring and maintaining it. In earlier
decades, smaller and less expensive multiuser
computers called minicomputers were
employed in many businesses, but minicomputers
are no longer widely used.