A firewall is a security mechanism that is designed to keep intruders from gaining access to your computer or its files. A firewall can protect your computer by denying access to intruders who try to access your computer through the Internet. It can also keep a log of the times someone tries to enter your computer system and can notify you when an unauthorized entry is detected.
For home users, a firewall can be a piece of hardware with configured security software, a software program running on an individual computer, or a combination of both. Either option allows you to define what type of things you want to restrict when on the Internet. Some firewalls can regulate your outbound traffic or what types of services you will allow access to through your computer when on the Internet.
As shown in the example below, without a firewall, there is nothing between your computer and the rest of the Internet to protect you from intrusions. Although the chances are slim that you will be the target of an unscrupulous intruder, the possibility is there.
In the example shown below, the addition of a hardware firewall adds security to your Internet connection. As mentioned above, your firewall can inform you of any intrusion attempts made on your computer system.
In this third example, you can see how a firewall can protect multiple computers. In this example, the firewall also acts as a hub, which allows multiple internal connections (computers) to access the Internet through the firewall. If your firewall does not have the ability to act as a hub or as a switch, you will need to purchase these items separately and install them between the computers and the hardware firewall.