A server is a a software program, or the computer on which that
program runs, that provides a specific kind of service to client
software running on the same computer or other computers on a
network. The client-server model is an architecture (i.e., a
system design) that divides processing between clients and
servers that can run on the same machine or on different
machines on the same network. It is a major element of modern
operating system and network design. The client provides the
user interface, such as a GUI (graphical user interface), and
performs some or all of the processing on requests it makes from
the server, which maintains the data and processes the requests.
An example is a web server, which stores files related to web
sites and serves (i.e., sends) them across the Internet to
clients (i.e., web browsers) when requested by a user. By far
the most popular web server program is Apache, which is claimed
to host more than 68 percent of all web sites on the Internet.
As is the case with other serve