StarChild from NASA defines and describes astronomy topics in a simple and easy-to-understand manner. The website is presented in two levels which cover the same topics: Solar System, Universe, Space Stuff, and Glossary.  Included are short videos, pictures, audio content, songs about the Doppler Shift and the Hubble Space Telescope, and many student activities.   There is an extensive section with links to activity books and posters, a question of the month with archives of previous questions, and links to other sites about astronomy, including the NASA astronomy site for high school students. 

courtesy of netTrekker