From the website:

VyOS is a community fork of Vyatta, a Linux-based network operating system that provides software-based network routing, firewall, and VPN functionality. How it's different from other router distros?

• Unified command line interface in the style of hardware routers.
• Scriptable CLI
• Stateful configuration system: prepare changes and commit at once or discard, view previous revisions or rollback to them, archive revisions to remote server, execute hooks at commit time...
• Image-based upgrade: keep multiple versions on the same system and revert to previous image if something went wrong.
• Not just firewall and VPN: includes routing protocols such as BGP and OSPF and complex routing policy language.
• Runs on physical and virtual platforms alike: small x86 boards, big servers, KVM, Xen, VMWare, Hyper-V...
• Completely free and open source, with documented internal APIs and build procedures.
• Community driven: patches are welcome, all code, bugs, and nightly builds are public.

For more information: http://wiki.vyos.net/wiki/Main_Page