The most usefull link I''ve found so far for explaining what Quaternions are and how they work is...

http://www.gamedev.net/reference/programming/features/qpowers/

However I have yet to find any programs showing an example of how they work. I use them myself even though I don''t understand them. Despite what others say Quaternions are very usefull and they work great! However, if another simpler solution will work just as good use that solution.