PWM stands for "pulse-width modulation". It is a way of simulating an analog signal by varying the duty cycle of a digital output.
For example, to control backlight intensity, we take a square wave output running, say, at 1kHz, and vary the time that the signal is high from 0 to 100% (in this example, from 0.0 to 1.0 ms). We then run the square wave through an RC (resistor-capacitor) network to smoothe(integrate) the digital output into a relatively stable analog signal suitable for control of variable-voltage devices.
We use PWM signals in our systems to control backight intensity(brightness) and Vee (the contrast control voltage for passive displays).
We don't have a technical glossary, but http://www.whatis.com and other online resources can help you to get up to speed with industry terms. You can also run an ADS forum search for a term to see the contexts in which it is used.