I wrote this genre of the moment at the request of Dave from Oklahoma Lefty.
Alternative country, also known as Alt-Country (and sometimes called insurgent country) is a subgenre of country music but it is not very similar to popular country music. Musically, it is basically a mixture of bluegrass, rockabilly, folk rock and punk. To me, the idea of mixing punk and country music sounds a bit strange but in some cases, it really does work. It most cases, the lyrical content is more punk than country while the music itself is more country or bluegrass, or in some cases, folk-sy.
The term alternative country started in the 1990s to describe...well, basically any artist that did not follow the aesthetics of mainstream country music. However, even though the term alternative country did not come about until the 1990s, Jason and the Scorcher's, which is one of the first bands to try and create a hybrid of country and punk, formed in 1981.
To me, alternative country incorporates the hillbilly sound of country, with punk lyrical content and a DIY (Do It Yourself) attitude reminescent of punk music. Bands that are a part of this subgenre are not the typical commericalized country music that is played on the radio these days. Personally, I would think that if Johnny Cash had started making music in the 1990s instead of when he did, he would probably be considered alt-country. That's just my opinion though, however, Wikipedia does put Johnny Cash in their list of alternative country musicians (then again, they also put both Gaslight Anthem and Kings of Leon in that list and that doesn't make a whole lot of sense to me).
To me, the line between folk punk and alt country is a very fine one at times.
Here are some websites where you can learn more about the genre:
What is Insurgent Country?
Here is a list of some alternative country artists/bands:
Jason and the Scorchers
500 Miles to Memphis
Band of Horses
Drag the River
Nine Pound Hammer
Drive By Truckers