From the illumos developer’s guide:
“illumos is a consolidation of software that forms the core of an Operating System. It includes the kernel, device drivers, core system libraries, and utilities. It is the home of many technologies include ZFS, DTrace, Zones, ctf, FMA, and more.
We pride ourselves on having a stable, highly observable, and technologically different system. In addition, illumos traces it roots back through Sun Microsystems to the original releases of
The illumos project provides several online books for developers: