Virtuozzo is a proprietary system and control panel used to host a VPS, much like cPanel/WHM is used for shared and reseller hosting. It provides an administrative side with a sophisticated control panel, and also provides the individual users with a more limited set of functions on their own "Power Panel".
Virtuozzo is used to manage your container. With it you can start, stop, reboot, reload and do many other things to the container itself. You can also view resource usage, restart services, and install new software.
You will still, however, need to use cPanel/WHM or Plesk to create new accounts and add new domains to your account. Virtuozzo is only for maintaining the container; cPanel or Plesk is still used for most day to day hosting operations.