Doug Burns has a great video presentation up at the Pythian blog, in which he gives a great explanation of the basics of Oracle's parallel execution feature.

It's a great video and it got my head straight on a lot of things (and just created more work for objects that I need to clean up!). One caveat is that, for me anyway, the video seemed to progressively lag behind the audio as the presentation went on. Slightly annoying, but still worth the watch.