A firewall is a part of the computer system or network that is designed to block unauthorised access to a network. It ensures that only legitimate users are allowed to access a particular network.
The firewall controls traffic coming into and leaving the system by permitting authorized communication. It hides the users home network from the outside world.
One major thing you should know about firewall is that it can be a software or a hardware. Apart from being a separate hardware, there are some firewall features that are also built into any network.
The firewall also monitors all the requests coming into the system, alert the user, and asks permission for allowing or blocking them as well.
There are some clear examples of firewalls such as Windows firewall (check securing Operating system module), Norton Personal Firewall, McAfee Personal Firewall, Sunbelt Personal Firewall, Zone Alarm and Comodo Personal Firewall among others.