The OpenSolaris Project consists of three key elements: (1) the OpenSolaris source code, (2) the OpenSolaris developer community, and (3) a website for collaboration: opensolaris.org. The Solaris OS, however, is Sun's operating system product, and future versions of Solaris will be based on technology from the OpenSolaris project. Solaris is available as a free binary download, and Sun offers service packages and regular updates. So, Solaris is a product supported by a company; OpenSolaris is a development project run by an community of developers.
There are several related items that need to be understood:
Solaris Express: This is a binary release for customers. It's Sun's official release of the OpenSolaris bits as well as additional technology that has not been released into the OpenSolaris source base. Sun offers limited support for this release. It's primarily intended for Solaris customers to try out the very latest technology that will eventually be productized by Sun. Solaris Express is updated monthly and is available as a free binary download.
Solaris Express Community Release: This is a binary release for developers. It's Sun's latest, unsupported release of the OpenSolaris bits as well as additional technology that has not been released into the OpenSolaris source base. Developers can build the OpenSolaris source by using this release as the base system. The release also has a code name -- Nevada -- and it's updated every two weeks.
OpenSolaris: This is the source base for Solaris development. Currently, it consists of several technology components called consolidations. See the Downloads Page for details on the technologies that have been released, and the OpenSolaris Roadmap for the schedule of upcoming technology releases. At present, the OpenSolaris source base is not enough to bootstrap an entire system, so developers start by downloading an OpenSolaris distribution and installing the OpenSolaris bits on top.
In Summary: Customers concerned about stability should use Solaris 10 since that's the company's officially-supported enterprise product. Customers interested in trying out the latest Solaris technologies within an official binary distribution are free to use Solaris Express. Developers working in the OpenSolaris community should use Solaris Express Community Release as the base on which to build the OpenSolaris source.