Radio waves are radio waves, the difference is that the message sent over the radio waves with HDTV is a digital signal and with analog TV it is an analog signal. Digital signals can often tolerate "noise" better than analog signals. Often analog signals are corrupted by "noise" which is often from other nearby signals and other electronic devices emitting radio waves. In an analog signal, "noise" will show up as "snow" or distortion on the picture, but the digital receiver will just discard the noise. The digital signal knows which 0s and 1s belong to it and just throw away the rest. An analog signal will show anything that comes across that certain frequency, which is why the picture is often imperfect. Amplifiers increase signal strength, but they also increase the "noise" in the reception, as well, because they are analog devices and they too will amplify anything that comes across the frequency.
