Firewall Your Network
A Firewall basically sits in between your Internet connection (dial-up modem, cable-modem, DSL, T1, etc) and works by directing all the internet activity using a set of rules.
The default rules, ideal for most users, are essentially simple in nature. They allow you to "surf" to the outside world and visit web-sites, FTP, email and so forth. And as you go about your tasks on the Internet, a good firewall allows return traffic from those tasks, that you requested, to pass through. If, however, some other TCP/IP traffic comes in, requesting information from your computer, and that traffic is not in response to your requests, a good firewall refuses to respond, and logs that attempt. Thus, you are allowed to go about your normal business, but when the bad guys try to come after you, they are stopped cold. Think of a firewall as your friendly traffic cop down on the corner, making sure that things travel smoothly, and enforcing good rules on your internet traffic.