What is Enlightenment?
Enlightenment is a window manager. Enlightenment is a desktop shell. Enlightenment is the building blocks to create beautiful applications. Enlightenment, or simply e, is a group of people trying to make a new generation of software.
As far as an operating system, Enlightenment DR-17 (e17) is offered as the default desktop on a several. Yellow Dog being the one for PowerPC linux. That does not mean that one can not add e17 to their operating system of choice.