What is Firewall

A firewall helps to keep your computer more secure. It restricts information that comes to your computer from other computers, giving you more control over the data on your computer and providing a line of defense against people or programs (including viruses and worms) that try to connect to your computer without invitation.

You can think of a firewall as a barrier that checks information (often called traffic) coming from the Internet or a network and then either turns it away or allows it to pass through to your computer, depending on your firewall settings.

Firewall is turned on by default. (However, some computer manufacturers and network administrators might turn it off.)

How does it work?
When someone on the Internet or a network tries to connect to your computer, we call that attempt an "unsolicited request." When your computer gets an unsolicited request, Firewall blocks the connection. If you run a program such as an instant messaging program or a multi player network game that needs to receive information from the Internet or a network, the firewall asks if you want to block or unblock (allow) the connection. If you choose to unblock the connection, Firewall creates an exception so that the firewall won't bother you when that program needs to receive information in the future.

For example, if you are exchanging instant messages with someone who wants to send you a file (a photo, for example), Firewall will ask you if you want to unblock the connection and allow the photo to reach your computer. Or, if you want to play a multi player network game with friends over the Internet, you can add the game as an exception so that the firewall will allow the game information to reach your computer.

Although you can turn off Firewall for specific Internet and network connections, doing this increases the risk that the security of your computer might be compromised.

What Windows Firewall does and does not do

Helps block computer viruses and worms from reaching your computer. Detect or disable computer viruses and worms if they are already on your computer. For that reason, you should also install antivirus software and keep it updated to help prevent viruses, worms, and other security threats from damaging your computer or using your computer to spread viruses to others.
Ask for your permission to block or unblock certain connection requests. Stop you from opening e-mail with dangerous attachments. Don't open e-mail attachments from senders that you don't know. Even if you know and trust the source of the e-mail you should still be cautious. If someone you know sends you an e-mail attachment, look at the subject line carefully before opening it. If the subject line is gibberish or does not make any sense to you, check with the sender before opening it.
Create a record (a security log), if you want one, that records successful and unsuccessful attempts to connect to your computer. This can be useful as a troubleshooting tool. If you want Firewall to create a security log, see Enable security logging options. Block spam or unsolicited e-mail from appearing in your inbox. However, some e-mail programs can help you do this. Check the documentation for your e-mail program to learn more.

Hardware Firewall

A hardware firewall is a piece of equipment that you physically connect to your network. It monitors all aspects of the communications that are sent and received, and inspects the source and destination address of each message that it handles. This provides security in helping to prevent intrusions to your computer or network. A software firewall performs a similar function by using a program inside your computer instead of an external device.
A firewall is a protective boundary between a computer (or a network of computers) and the outside world, where people or programs might try to access your computer without invitation. Hackers commonly use software that scans the Internet looking for computers with unprotected Internet connections. This software sends a very small message to your computer. If your computer does not have a firewall, your computer will automatically answer the message and thus announce that it is unprotected. But if you have set up a firewall, it will receive such messages and not answer them, so hackers won't even know that your computer exists.