Some radio stations that you can hear on FM, AM, satellite radio, etc.... are also broadcasted online. BUT NOT every one is.
There are also radio stations which are only broadcast online (meaning you wouldn't find them elsewhere other than on the internet).
In total, there are a ton of them, but not every single off internet radio stations can also be heard on the internet.
Here are just a very few online radio sites/stations
www.accuradio.com Accuradio
http://music.aol.com/radioguide/bb AOL/CBS Radio
http://new.music.yahoo.com/launchcast/ Yahoo Radio
http://pandora.com/ Pandora (one of some of the internet radio sites that allow you to make customized stations)
etc....
EDIT - I didn't realize yahoo radio is now also associated with CBS. So both Yahoo and AOL radio have the same/all CBS stations plus their own stations, but AOL has a lot more of their own personal stations than yahoo has.