This website is chock full of info about composers, genres like opera that you can learn about, and even a podcast to listen to. It's super important to know the background to the music you're making - it can inform the way you play in so many ways. The lucky thing is, it's fun to learn about this stuff. There's an A-Z composer index to look up all the gossip on the big name composers you might be playing the pieces of.
Go to the website: http://www.classicsforkids.com/