NHK just released an iPhone Radio app, "Rajiru" that you can listen to the NHK Radio programs while you're in Japan.
Unfortunately, you cannot listen to the stream if you are outside Japan.
The R1 is their AM Ch.1 which include Japanese news and talk programs. The R2 is their AM Radio Ch.2 which mainly airs educational programs. The FM channel mainly broadcasts music programs.
Mainly their programs are in Japanese. It may not be useful for non-Japanese speakers in Japan.
However, this is a must-install iPhone app for everybody since the R2 becomes English channel in case of the natural disaster. It's good to remembe just in case.
Rajiru is also available online via web browser (but restricted to the people in Japan).
If you would like to listen to private radio stations, go to Radiko website, iPhone or Android app. Listen to Japanese radio and practice your Japanese skills. (Radiko service is also restricted in Japan)