it is just mordern adaption of the old midever security 
standby.
the firewall configuration has two components .To routers 
that do packet filtering and application gateway.The 
advantage of their design is as every packet must transit 
to filters and an application gateway to go in or out.

A firewall is a set of related programs, located at a
network gateway server, that protects the resources of a
private network from users from other networks. (The term
also implies the security policy that is used with the
programs.) An enterprise with an intranet that allows its
workers access to the wider Internet installs a firewall to
prevent outsiders from accessing its own private data
resources and for controlling what outside resources its own
users have access to.

its like antivirus

firewall is a software program or a hardware device which 
controls the network traffic.