Comodo Firewall monitors incoming and outgoing internet connections from your PC to make sure that legitimate traffic gets through, and malware is consistently blocked.
Comodo describe their policy here as "Default Deny Protection": unless they know a program is safe, it's not allowed to communicate online, or use another program to do the same (by opening an Internet Explorer window, for instance). While this is certainly secure, it also makes for plenty of alerts early on, as the program is regularly warning you about one program or another. The firewall will by default remember your answers, though, so you're only asked once per program activity and the pop-ups soon slow down.
Comodo Firewall's Defense+ technology adds a host of additional security features. There's a Sandbox, allowing you to run potentially dangerous programs in an isolated environment where there's little chance they can harm your PC, for example. You're able to ask the program to protect specific Registry keys, files or folders against unauthorised changes. And a "Blocked Files" module lets you lock down particularly sensitive files and folders so nothing can access them.
If you're a PC security novice then there's no need to worry about how any of this works. You can just install Comodo Firewall, accept all the default settings and leave it to get on with protecting your system.
If you're more experienced, though, you'll appreciate the program's extreme configurability. Comodo Firewall comes with a host of tweaks and options to change just about every aspect of its operations: you can set up your own application rules, custom network policy, firewall rules and settings, Defense+ policy and more, fine-tuning your security to the precise levels you need.
New features this time start with the Viruscope. This looks for programs carrying out potentially unwanted actions - setting themselves up to run when your system starts, for example - and not only alerts you, but can undo any damage, restoring your system to normal.
The Website Filtering module is another welcome addition, vetting the links you click for safety. It can also limit access for particular users, perhaps preventing your children from viewing inappropriate web content.
A new Protected Data Folder gives extra protection to sensitive files, ensuring they're completely invisible to any sandboxed applications.
And there are plenty of other significant enhancements elsewhere, from a 700% speed increase for the sandbox, to a range of time-saving interface tweaks, and more than 300 bugs fixed since the previous edition.