To use the operating system, just download and install one of the distributions. OpenIndiana is a general-purpose system suitable for users and developers. OmniOS is aimed squarely at server use. Joyent has SmartOS for hosting applications and virtual machines, and the illumos-live build system that constructs a bare-bones system image.
It is run by the community on a system of meritocracy. Multiple community groups and interests (including people like you!) are stakeholders in the project. Distributions like BeleniX, NexentaStor, OpenIndiana, and SmartOS are moving to using illumos as their core, and other distributions have shown interest as welland anyone and everyone is welcome to contribute.
How do I start contributing?
OpenSolaris was historically difficult to build as a distribution because it was assembled by many separate teams at Sun. The illumos community distributions are working to make this easier.
Is illumos compatible with Solaris/OpenSolaris?
Yes! We highly value binary compatibility for applications and drivers. If your application runs on Solaris but not on illumos, then tell us, because it's a bug!
Note that we can only claim Solaris compatibility for versions of Solaris released before the fork. Solaris 10 update 11, and Solaris 11, may contain incompatible changes.
What changes does illumos maintain?
We started off with the codename "FreeON", and later realized it is used by an existing project. We finally settled on illumos, after many hours of suggestions and counter-suggestions. illumos (pronounced i-llu-MOS and written in lowercase) ties in with Sun and light. It's the closest to ON we could get!