The following description of OpenStreetMap is "upgoer5 compliant" http://splasho.com/upgoer5/ It uses only the 1000 most commonly used words of the english language to explain our project.
We want to get people to draw where things are all over the world on computers. Anyone can use and change it the way they want. You can see lines in a picture and draw new ones to mark the roads and trails you know (with their names and types and anything else you can actually see on the ground, and points where they come together or cross). You can mark positions or areas of places one would find outside (a park, a college, the corner coffee shop, or even a little drinking water spot you found on the side of a building after a long walk) and what they have that you can see and explain. With these known, you and others, whether by car, bus, train, or even horse, can get easy directions (or harder/longer if one is in such mood to see other sights). You can put the things on whatever you want, like bags or shirts or even a hair brush. People only have to say our name in return for getting or showing the changes that others have made, and they will need to allow others the same rights to the drawing changes they make to the world for free. Would you help us draw the world?
This is a wiki so we can collaborate on making this description longer and more detailed. Feel free to edit, however it must remain upggoer5 compliant (paste it into the tool to check)
This just a bit of fun, but it might also be an interesting exercise in trying to explain our complex project using non-technical language. There have been many attempts to write a quick intro to OpenStreetMap, e.g. on pages such as About and Using OpenStreetMap.