From a code point of view, Equinox is an implementation of the OSGi R4 core framework specification, a set of bundles that implement various optional OSGi services and other infrastructure for running OSGi-based systems.
More generally, the goal of the Equinox project is to be a first class OSGi community and foster the vision of Eclipse as a landscape of bundles. As part of this, it is responsible for developing and delivering the OSGi framework implementation used for all of Eclipse. In addition. the project is open to:
As part of the RT project, the Equinox OSGi code is managed by the RT PMC. There is a close binding however to the Eclipse project and its PMC. Equinox, as the base for all of Eclipse, ships with all the major releases. The various other bundles developed here may ship independently and on different schedules.
The easiest way to get involved in Equinox is to try it out. See the Quick Start Guide for how to get up and running with the standalone Equinox OSGi framework and the downloads page for more information on acquiring various Equinox OSGi bundles.
If you want to find out more, ask questions, report bugs, get (or contribute :-) code, check out the community resources and look into the work going on in the framework, bundles or incubator components of Equinox.