A firewall is a software program or piece of hardware that blocks fraudsters from entering and using your computer. Fraudsters search the Internet in much the same way that some telemarketers automatically dial random phone numbers. Fraudsters send electronic probes, or pings, to thousands of computers and wait for responses. Firewalls prevent your computer from responding to these random pings. A firewall blocks communications to and from sources you don't permit. This is especially important if you have a high-speed Internet connection, such as DSL or cable.
Some operating systems have built-in firewalls that may be shipped in the "off" mode. Therefore: