A firewall is either a device or a software package that blocks user defined or system defined ports preventing inappropriate content, such as viruses, spyware, and popups to be sent or received by the computer. A firewall is best used on a high speed connection such as DSL or wireless, but can also be used by a dialup connection. 

Firewall software is being packaged with security suite software such as Norton or the Mcafee Security suites.

Information about Different kinds of firewalls:

1. ZoneAlarm: www.zonealarm.com
2. Windows XP built in firewall: WWW.Microsoft.com/windowsxp
3. Sonic Wall: www.firewalls.com
4. Black Ice: http://www.digitalriver.com/dr/v2/ec_dynamic.main?SP=1&PN=10&sid=26412
5. Symantec Norton Security Suite: www.symantec.com
6. Mcafee Security suite: http://us.mcafee.com/