HD Radio or High Definition Radio provides better quality than the normal FM or AM reception. To receive HD Radio, you would of course would need to have a HD Radio Receiver as well. From what I know, this high definition radio is able to do multicast which means it is able to transmit multiple new local radio channels on the same frequency. Another that I find it interesting is that it is able to do iPod tagging. Listeners are able to 'tag' songs just by pushing a button. When you iPod is synced to the iTunes next time, tagged songs will be displayed for you to purchase.
By the way, HD Radio is not only limited to your car but you can even use it in your home. Once everything has been set up, you can listen to it just like a regular radio. In New York alone, there are about 90 High Definition Radio stations. Of course this is not just limited to New York alone. You can easily find HD Radio stations else where too. If you live in Texas, just scan through for the Texas High Definition Radio stations. I would love to get one but I'm the number of HD radio stations in Malaysia are limited.