This Essentials of Music website is a great place to get a brief overview of a variety of musical information. They have three primary sections on the site: Eras, Composers, and Glossary. The Eras section contains separate sections for: Middle Ages, Renaissance, Baroque, Classical, Romantic, and 20th Century. Clicking on one of these leads you to further information about the Historical Themes, Musical Context, Style, and Composers of that era.
The Composers section includes a brief biographical sketch of the composer, a few musical examples you can listen to, and a list of works by that composer. The Glossary is a wonderful collection of all sorts of musical terms with little excerpts to further illustrate some of the terms. A wonderful resource for teachers and students alike!