At EclipseCon 2011 there was a great introductory presentation on p2. It gives a nice overview of the whole provisioning system. You can find it recorded here(video).
This blog post provides some background on why p2 was created as well as a brief overview of what p2 repositories are and how this relates to a runtime.
This presentation(slides only) sheds light on more advanced p2 features and turns our attention to its extension points.
Finally, the p2 wiki also provides both getting started guides as well as information on more advanced features.