This site would be a great resource for anyone doing a self-study of classical music, or someone teaching classical music without extensive training in that area (can we say Home schooler, boys and girls?)

Essentials of Music contains brief biographies of 60plus composers, overviews of the 6 main eras of classical music and a glossary of 200 musical terms, many with audio examples.

The site is maintained by the sellers of essential classics, but the info is FREE.