What is a firewall?
A firewall is the first line of defense when protecting private information. A firewall acts as a protective barrier between your computer and the internet, monitoring all incoming and/or outgoing traffic and allowing only the network traffic you permit. Firewalls come in the form of software, which nestles itself between your operating system and your network card. They also come in the form of hardware; for many home and small office users, it is a simple router device that sits between your computer's network jack and the wall connection. You can customize the level of protection the firewall gives you, setting it to filter information flow from specific domain names, addresses or types of network traffic.

What does the firewall do?
A firewall examines all traffic routed between the two networks to see if it meets certain criteria. If it does, it is routed between the networks, otherwise it is stopped. A firewall filters both inbound and outbound traffic. It can also manage public access to private networked resources such as host applications. It can be used to log all attempts to enter the private network and trigger alarms when hostile or unauthorized entry is attempted. Firewalls can filter packets based on their source and destination addresses and port numbers. This is known as address filtering. Firewalls can also filter specific types of network traffic. This is also known as protocol filtering because the decision to forward or reject traffic is dependant upon the protocol used, for example HTTP, ftp or telnet. Firewalls can also filter traffic by packet attribute or state. 

Who needs a firewall?
If you connect to the internet you want to protect yourself then you need a firewall set on your computer.  Anyone who is responsible for a private network that is connected to a public network needs firewall protection. Furthermore, anyone who connects so much as a single computer to the Internet via modem should have personal firewall software. 

What are the risks of not having a firewall?
If your computer is automatically set to enable file sharing or to keep network ports open while you are online, you could be susceptible to a variety of attacks. If you don't have a firewall, which will monitor ports to stop unwanted traffic from slipping through, you have to know how to manually close ports and disable file sharing in order to control risky traffic from coming in to your computer.

How do I get a firewall?
In Windows 7, Vista, and XP, software firewalls are built into the operating system. Earlier versions of Windows did not have firewalls built in. Macintosh computers running Mac OS X 10.2 and later are also equipped with a built-in firewall. You can purchase a third-party firewall package.  There are a number to choose from, such as Zone Alarm, Norton Personal Firewall, Tiny, Black Ice Protection, and McAfee Personal Firewall. Many of these software companies offer free versions or trials of their commercial versions. Even home and small office broadband routers have rudimentary firewall capabilities built in. These tend to be simply port/protocol filters, although models with much finer control are available.