A great resource for pupils and teachers of Music put together under the auspices of Learning & Teaching Scotland. The A - Z Dictionary of concepts with audio samples is impressive. However, there is much more than I had realised on offer e.g. short essays (also with audio illustration) featuring concepts, photographs of composers, instruments etc. From the index you'll be able easily to navigate your way to the various areas. 

Visit http://www.ltscotland.org.uk/nq/resources/learnlisteningonline/index.asp to find out more.