A firewall is security program installed on a gateway, server or proxy to prevent outside users accessing the private network. Many intranets use firewalls to protect its resources like the database and other important documents from outside users and at the same time allowing its users to access other networks.
Firewalls often installed on a proxy server, also sometimes block requests from other sites to access the private data on the system. Firewalls, basically examine the network packets to determine the destination of these packets and whether they should be allowed access to the network or not. Often requests from predetermined requests can be allowed to access the network. Many companies develop firewalls and there are also free firewalls applications designed to protect the personal computer of an individual.