basically when you log onto the net you are given an IP address. with this you become another node on the network and just as you access other machines (like Greg's server here) people can potentially access your machine.

There are a lot of ways to access it, through a lot of different "ports" or sockets. And someone who is malicious could potentially get into your machine and take your files, passwords, information. You may not believe this, but Windows is not very secure.

The idea of a firewall is to sit between your machine and your cable modem. This can either be a hardware firewall (in my case) or a software firewall (such as Zone Labs). They will intercept incoming IP packets and determine if they should be there. Such as a connection request to a web/email/news server, or a regular data packet and block what should not be there.

@Home would do this very frequently. They wanted to ensure that you were not running a web server from your home, so they would intermittently ping port 80 (the web server) and, if they got a response, would see that your running a web site from home (against the rules).

These days it is good to run one. Zone labs is a good product and does a great job in blocking the malicious attacks. It will slow down the machine slightly because all incoming packets are going through this firewall. But without it, it could potentially be worse.