I enjoyed Richard Weait's 30 minute introduction video to OpenStreetMap. It covered what I thought were the salient points as I understand them. I also like the video editing... keeps things interesting.
If you prefer a 5 minute version, here is one from Steve Coast at Ignite Spatial NoCo.
I imagine that there are more such summary presentations out there, but they are not easy to find. I am always on the lookout for these kinds of presentations to help me understand concise ways to communicate "what is OSM" to the people I meet.
Please comment if you know of other helpful OSM introductory summaries.