Network security is a complicated subject, historically only tackled by
  well-trained and experienced experts. However, as more and more people become
  ``wired'', an increasing number of people need to understand the basics of
  security in a networked world. This document was written with the basic
computer  user and information systems manager in mind, explaining the concepts
needed to  read through the hype in the marketplace and understand risks and how
to deal  with them.
Network security is becoming more and more important as people spend more and
  more time connected. Compromising network security is often much easier than
  compromising physical or local security, and is much more common.
The networks are computer networks, both public and private,
  that are used every day to conduct transactions and communications among
  businesses, government agencies and individuals. The networks are comprised of
  "nodes", which are "client" terminals (individual user PCs) and one or more
  "servers" and/or "host" computers. They are linked by communication systems,
  some of which might be private, such as within a company, and others which
might  be open to public access. The obvious example of a network system that is
open  to public access is the Internet, but many private networks also utilize
  publicly-accessible communications. Today, most companies' host computers can
be  accessed by their employees whether in their offices over a private
  communications network, or from their homes or hotel rooms while on the road
  through normal telephone lines.
Network security involves all activities that organizations,
  enterprises, and institutions undertake to protect the value and ongoing
  usability of assets and the integrity and continuity of operations. An
effective  network security strategy requires identifying threats and then
choosing the  most effective set of tools to combat them.

Article source:http://www.cc.boun.edu.tr/network_security.html