AM Modulation shows you the wave and its spectrum. wc is the carrier frequency, wm the modulator frequency, m is the amplitude of the modulator (you can't change the offset). If you check "overmodulate" the scale of m is extended to allow overmodulation.
On my browser there is a strange effect when another window overlaps this window: the values of m etc start changing on their own. Does this happen for you, can you figure out what causes it?
Same thing for FM modulation
... and if you have not yet tried your hand at Fourier synthesis and its sound, here is a good one to play with, up to the 15th harmonic. On the other hand, if you want to see just a square wave synthesized with up to 64 harmonics (and the Gibbs phenomenon) this is the site for you. (To work the applet at the bottom of that page you have to type in a bit stream, like 01010 and hit one of the display bars.)