From their website:

Symphony is a distribution of GNU/Linux based on the Debian and KNOPPIX GNU/LINUX operating systems. Symphony will do things a bit differently than other Linux operating systems; making it easier to use and more intuitive than most existing distributions.

http://www.symphonyos.com/about.html

Mezzo is the revolutionary desktop system built for Symphony. Based on the concept by Jason Spisak this system simplifies and organizes the desktop environment in such a way that practically anyone can feel comfortable using it.

Mezzo disposes of standard conecepts like "The desktop is a folder" and nasty nested menu systems that are hard to navigate and harder to manage and instead presents all needed information directly to the user via the main desktop and four desk targets for tasks and files related to System, Programs, Files, and Trash.