Musical Periods
Britannica Encyclopedia http://school.eb.com/levels/middle/article/275994
Classics for Kids http://www.classicsforkids.com/shows/collections.asp
Baroque Period
About the Baroque Period http://www.baroque.org/baroque/
Baroque Music from Wikipedia http://en.wikipedia.org/wiki/Baroque_music
Music History 102: Baroque Music http://www.ipl.org/div/mushist/bar/
Classical Era
Music History 102: Classical Era http://www.ipl.org/div/mushist/clas/
Classical Period - Wikipedia http://en.wikipedia.org/wiki/Classical_period_(music)
Romantic Era
Musical History 102: Romantic Era http://www.ipl.org/div/mushist/rom/
Romantic Music from Wikipedia http://en.wikipedia.org/wiki/Romantic_music
Modern or Twentieth-Century Music
Modern or Twentieth-Century Music http://www.ipl.org/div/mushist/#twen
Twentieth-Century Classical Music from Wikipedia http://en.wikipedia.org/wiki/20th-century_classical_music