What is a firewall?

One of the topics this week included firewalls.  What is a firewall?  Well here is what Microsoft says:

A firewall is a software program or piece of hardware that helps screen out hackers, viruses, and worms that try to reach your computer over the Internet.

... and nice graphic to go with it.

Even more detail:

A firewall is a system designed to prevent unauthorized access to or from a private network. Firewalls can be implemented in both hardware and software, or a combination of both. Firewalls are frequently used to prevent unauthorized Internet users from accessing private networks connected to the Internet, especially intranets. All messages entering or leaving the intranet pass through the firewall, which examines each message and blocks those that do not meet the specified security criteria.

So what controls cover firewalls? 

FAMILY: SYSTEM AND COMMUNICATIONS PROTECTION: SC-7 BOUNDARY PROTECTION

For home users there is Shields Up!  I have used this site for years and it provides a number of ways to test your firewall (your router firewall).  The following is an example output from a sample scan:

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GRC Port Authority Report created on UTC: 2015-05-18 at 23:57:39
Results from scan of ports: 0, 21-23, 25, 79, 80, 110, 113,
                            119, 135, 139, 143, 389, 443, 445,
                            1002, 1024-1030, 1720, 5000
    0 Ports Open
    0 Ports Closed
   26 Ports Stealth
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   26 Ports Tested
ALL PORTS tested were found to be: STEALTH.
TruStealth: PASSED - ALL tested ports were STEALTH,
                   - NO unsolicited packets were received,
                   - NO Ping reply (ICMP Echo) was received.
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