PWM has a fixed frequency and a resolution limited by the clock cycles per PWM cycle.With Sigma-delta the input value is summed to an error register and the output subtractsfrom that register - in effect there is a feedback loop trying to keep the error within bounds,and the input resolution is decoupled from the output resolution.You can use single-bit output, in which case the proportion of ones in the output (averagedover time) is proportional to the input value (which can be 16 or more bits if you want). Thefrequency of the output is not constant and usually the signal needs converting to analogwith a very tight-spec charge-summing stage (the precise timing of the clock edges is all-importantin getting good linearity).But you can also use PWM as the output, say 8-bit PWM out, 16 bits in, and the error valuehas its top 8 bits pushed out to the PWM every PWM period - the lower order bits are stillaccounted for and provide high resolution at low bandwidth.In general sigma-delta trades bandwidth for accuracy (clock has to run a lot faster than thesample rate).Real converters use more tricks and are more complex, but the basic idea is to maintainan error value and use negative feedback to keep the accumulated error small.
After looking online and reading through some complicated terminology and mathematics, I still can not effectively distinguish between PWM and Sigma-Delta DACs. I know that Sigma-Delta is faster, more accurate and efficient (but why? How? How is the sampling different, and what does each do that makes them so different?)
A DAC generates a true DC analog output voltage directly. A PWM outputs a digital signal of fixed frequency and a variable duty cycle. They are two drastically different output signals used in different ways. A PWM signal can be made to generate a true DC analog output voltage if followed with a proper passive or active low pass filter. Lefty