In the computer world, the purpose of the firewall is to stop unfriendly or unwanted access to a computer or computer network. The firewall is the second line of defense for the computer user. A good internet service provider is the first.
Most service providers will furnish their subscribers with a small measure of protection. This is especially true concerning email traffic. Most email routed through an internet service provider to a user account is scanned before being forwarded on to your computer. So, in essence, this is your first firewall. Do not depend on your internet provider to keep you safe. Most data is simply passed through their system to yours.
Should a hacker gets your internet address, the hacker will begin to attempt to access your computer. Frequently, this is done by feeding a software device through the internet to embed itself on your hard drive and relay information back to the hacker. Armed with you system information and maybe some of your passwords, the hacker is ready to do some damage by stealing your data or your identity or both.
You need protection close to home to stop this from happening. Your local firewall is your best hope. Most major operating systems come with firewall software already installed. If not, companies like McAfee produce some excellent firewall programs. For home use, the less expensive types of firewalls are usually more than adequate. If you own a company that deals in proprietary or secret materials, you will want to go after the big guns.
Once installed on your computer, firewalls come with settings of low, medium, and high with some grading along the way. This means that you have about 5 to 10 security levels to choose from. The manufacturer will almost always have the software preset at the level recommended for home use. If you turn it up too high, nothing will come through from the internet. You might as well disconnect it from your computer if you are that fearful.
Low settings will allow all of the awful stuff that roams the internet looking for open doors to stroll right on in. Something between the middle and high is usually a good place to start. If this is too restrictive, back it off a little at a time until you feel that your system has a comfortable balance between high and low security. When it fits the pattern that lets you have the internet access you desire, trust your other protection like anti-virus programs to do the rest.
Most routers come with a measure of hardware based firewall protection. They are not perfect because often the have little ability to be customized to fit the users needs. So, they knock out the big pieces to take some of the pressure off of your software. If you use a cable modem or DSL, you will want to get a router to add the extra measure of security because you are always on an open line to the internet.