Umm... Brad's doc =P
If you want the basic understanding:
"Square" channels are called such because they make a square-shaped waveform. Looks something like:
The 'height' of the generated wave is the Volume (discussed in the evelope/decay section of Brad's doc). The length of the generated wave is the Programmable Timer... or wavelength (determines the pitch).
"Triangle" channels are called so for the same reason as it generated a triangular shaped wave:
There's no volume control on the Tri channel (excluding the $4011 thing... which isn't widely used).
The "Noise" channel is very similar to square channels. They have a volume and a wavelength... however the wavelength triggers a random number generator... which makes the generated wave appear to be random (which produces a noisey effect).
That's the very basics so you can better understand the concept he's talking about in the doc. But really... that doc covers EVERYTHING you need to know. It may take a few reads and re-reads to fully understand... but just keep with it ;)