For learning about popular music, the Pandora website is a huge development, and the site isn't too shabby on country music old and new. Far more instructive than simply reading about honky-tonk or the Bakersfield sound, spend a few hours plugged into the Hank Williams and Buck Owens channels to really get a feel for the music and be exposed to other performers of the same style. Try creating a station based on an exemplary performer from each style of country you can think of and put them into a mix. Accounts are free, so why not?
Coupled with Pandora, the All Music Guide website gives the musically curious the ultimate one-two punch. Listen to it on Pandora, and read about it (or buy it, even) off of All Music. The reviews are remarkably accurate and comprehensive, and like Pandora, it's a free resource. Audio clips are included with many reviews, but the sound quality is poor.
Roughstock maintains a collection of country music history pages that can give you a quick idea of how the style developed. There are also links to some pretty cool videos.
Country Music Television operates a site that includes an interactive timeline. Checkout what happened on this day in country music history, or enter any other date to search their thorough event database.