Reference
Pulsewidth
Pulsewidth is easiest to explain with diagrams.
This waveform has a pulsewidth setting of 0.5:
This waveform has a pulsewidth setting of 0.25:
This waveform has a pulsewidth setting of 0.75:
As you can see, changing the pulsewidth has a dramatic effect on the waveform, and on the timbre of the sound produced.
LFO
There is an LFO dedicated to modulating the pulsewidth. You can set its speed and amount.
Envelope
The pulsewidth section also has an ADSR envelope. The 'PW Env Amount' parameter controls how much the envelope affects the pulsewidth.