It was originally broadcast by CBS from September 2006 to March 2008.
Interesting thing about this show is that after the first season, the network cancelled it. However, because its large fan base basically protested the network, it was given a 2nd season (although the 2nd season only last 7 episodes).
A 3rd "season" has been made, although it is not on television. The 3rd season is a 6-part comic book series. There are also talks about a movie being made.
Anyway, the first season is about the residents of a small town in western Kansas (the town is called Jericho) as they deal with the life after the United States is torn apart by multiple nuclear attacks that destroyed almost all of the major cities in the country.
The 2nd season (which I have not completed watching about) so far is about how the resident of Jericho deal with the new "legitimate" government of the United States, the Allied States of America.
It's a pretty good show. I'd recommend checking it out.