Firewall is a security system which protect the System from Intruders and hackers. Firewalls also safeguards the PC from offensive software that come to reside on the software to stole the user personal data.
It is a software programs or hardware tool which notes and filters the traffic that flow into the PC through Internet connection. They block the files which are harmful to the system. There are set of rules to connect such as who to connect to the internet, what kind of connections can be made to the internet, what kind of files may be transferred. These Rules are can be set up by yourself or by administrators to allow traffic to their web servers, FTP and Telnet servers, thereby giving the computer owners/administrators control over the traffic that flows in & out of the systems.