Firewall

A guardian who protects inner network from outside intruders.

A firewall is a software or hardware-based network security system that controls the incoming and outgoing network traffic by analyzing the data packets and determining whether they should be allowed through or not, based on applied rule set.

What a firewall does


Generally, a firewall just filters information coming through the Internet connection into your private network or computer system.
 * "Hey wait, you are a packet from 204.53.213.54 trying to access our network 22.78.50.21? What do you want?"

Links

 * http://computer.howstuffworks.com/firewall.htm