There is no radio function on the iPad like there is with iTunes on your PC.
For general streaming, you just can't beat Pandora.
For specific radio stations, my "go to" app is PocketTunes. With this, I can listen to radio stations from all over the world! It also has many of my local HD stations which saves me money since now I don't have to buy a separate HD tuner for my HiFi system in the Man Cave.
Another streaming app I like is 5-0 Radio Pro. Not only does it have streaming "radio stations" from all over the world, it also taps into police and fire scanners from all over the world! How cool is that?