What is a firewall?

A firewall helps users control how their computers exchange data with other computers on the Internet or local network.

"A firewall is a system that secures a network, shielding it from access by unauthorized users. Firewalls can be implemented in software, hardware or a combination of both. In addition to preventing unrestricted access into a network, a firewall can also restrict data from flowing out of a network."
www.intel.com/products/glossary/body.htm

Follow up:

What are ports? "Ports: A port is a logical component of the TCP connection. Learning more about ports will help you better defend your network by closing off ports and services which are not required. You can read more about ports particularly the well known ports at Internet Assigned Numbers Authority(IANA). Remember, if a port is open, even something like port 80 which you use to access the internet, if you can get out, then a Hacker can get in."

This quoted paragraph above is an explanation of what ports are which was referenced from: root777.com Which contains a glossary of important security terms.

Now that we have defined a firewall and ports we can get into the explanation of what a firewall is and what it does.

A firewall is essential to computers that are connected to a network. It keeps people, namely hackers, from stealing important files or information. Many times hacking is an inside job but can be done outside of a local network. When we talk about a firewall we are talking about a wall between a computer and another computer.

It is important to note that a computer network is composed of ports. These ports are used by different computer programs. When we block certain ports, certain programs will not be able to access a network. Most browsers use port 80 to access the website aspect of the World Wide Web. If we block this port it does not mean we are completely blocking the Internet. It means we are blocking a means of accessing webpages visually.

There are numerous programs that use different ports. A list can be found here. List Of Computer Ports.

I use the picture in this article to demonstrate what I mean by a firewall and ports. If our computer represents the larger wooden block and the holes in it represent ports we can easily understand a firewall. The firewall is the blocks that block the ports or "holes" in the larger wooden cube.

If certain ports are open and not blocked they are susceptible to certain attacks. To further our understanding of a firewall it is important to know why this information is important to a hacker. Hackers may use a method of identifying what ports are open on your computer by means of a "port scan". This means that a hacker will find all the holes in you computer to see all the possible means of attacking it.

What firewall should I use?

This is an interesting question because there are many pros and cons about firewalls. By default many people will have a firewall on that is supplied by Windows OS. This firewall may not be enough to fully protect your computer from many threats or ports that could be open. There is software that will identify, not only security issues of open ports, but whether you are being port scanned or not. It is suggested by some that the best firewall program is Zone Alarm. But as for the best free firewall program many people will side with Comodo.

Feel free to comment if the article helped and what firewall applications you have had success with.