what is the difference of Resolution bandwidth and video bandwidth in spectrum analy

What is the difference between resolution bandwidth and video bandwidth in a spectrum analyzer? How do spectrum analyzer measure spectral energy? Is it avarage power in each RBW?

A spectrum analyzer is nothing more than this:

The bandpass filter in the front end limits how much energy width (in frequency) gets to the power detector by setting the RBW ( and also the center frequency F0).

AFTER the power is detected, you can smooth it out (lowpass filter) with the VBW control.

VBW is only used for visualization.
RBW, on the other hand, strongly affects the measuring time and noise.