What is a Firewall?

What is a Firewall?
A firewall is a tool that monitors communication to
and from your computer. It sits between your
computer and the rest of the network, and
according to some criteria, it decides which
communication to allow, and which communication
to block. It may also use some other criteria to
decide about which communication or
communication request to report to you (either by
adding the information to a log file that you may
browse whenever you wish, or in an alert message
on the screen), and what not to report.