ZoneAlarm is a software firewall originally developed by Zone Labs, which was acquired in March 2004 by Check Point. It includes an inbound intrusion detection system, as well as the ability to control which programs can create outbound connections—the latter not available in the Windows XP Service Pack 2 firewall. , the newest release is ZoneAlarm Internet Security Suite 2009. ZoneAlarm was formerly known as Zone Labs, the usage of which is declining.

Features 
In ZoneAlarm, program access is controlled by way of "zones", into which all Computer networking connections are divided. The "trusted zone" generally includes the user's local area network and can share resources such as files and printers, while the "Internet zone" includes everything not in the trusted zone. The user can specify which "permissions" (trusted zone client, trusted zone server, Internet zone client, Internet zone server) to give to a program before it attempts to access the Internet (e.g. before running it for the first time) or, alternatively, ZoneAlarm will ask the user to give the program permission on its first access attempt.

True Vector Internet Monitor 
"True Vector Internet Monitor", also known as "TrueVector Security Engine", is the core of ZoneAlarm. It is a Windows service which runs as "vsmon.exe" in the processes list. This Monitors internet traffic and generates alerts for disallowed access.

Operating System Firewall 
Operating System Firewall, OSFirewall for short, is a feature in ZoneAlarm paid versions only. This monitors programs and generates alerts when they perform suspicious behaviors.

SmartDefense Advisor 
SmartDefense Advisor is a service only available in premium versions of ZoneAlarm that helps the user with certain types of alerts. This utilizes a large database of trusted program signatures in order to provide the user with further advice upon requests from the program in regards to allowing or denying access to the Internet.

Versions 

64-bit versions of the ZoneAlarm product line will be made available for Windows Vista in the first half of 2009.

ZoneAlarm Extreme: A comprehensive version of Internet Security Suite that includes ForceField. It is currently in undergoing beta testing, but is available for download.
ZoneAlarm: A freeware version which includes a web and local network firewall with outbound program control and port stealthing.
ZoneAlarm Pro: This offers a OS Firewall identity protection, pop-up ad blocker, adware detection, cookie blocker. It does not have Antivirus features.
ZoneAlarm Anti-Spyware: Includes all the features of ZoneAlarm Pro, as well as privacy and identity theft features.
ZoneAlarm Antivirus: Includes ZoneAlarm firewall and antivirus protection provided by Kaspersky Labs.
ZoneAlarm Internet Security Suite: Includes all the features of every product with additional IM security and an anti-spam filter engine provided by MailFrontier and a parental control. It is the most expensive ZoneAlarm product.

ZoneAlarm Force Field: A virtualization engine that shields the computer and personal data from Internet and computer-based threats. It includes several layers of protection to combat phishing, spyware and dangerous file downloads.

Discontinued versions 
ZoneAlarm Plus: Discontinued from late 2004 for having fewer features than ZoneAlarm Pro but selling for almost the same price.
ZoneAlarm Wireless Security: Discontinued on October 19, 2005, because its functions were included in all paid versions of ZoneAlarm from version 6 onwards.

References

Other websites 
Official ZoneAlarm Site
Official ZoneAlarm Blog
Official ZoneAlarm User Forum 
ZoneAlarm Bugs 

Software
Antivirus software