I have been reading a lot about Cosmological experiments such as Planck or WMAP that aim to investigate the CMB's power spectrum. I am unclear though as to what theses sky surveys actually measure, is it the distance to regions of temperature fluctuations?
They measure the spectrum of microwaves (with pretty good angular resolution) over the whole sky.
The spectrum represents the red-shifted black-body of the hot, early universe plus local contributions. They separate out the underlying black-body spectrum and that is the signal.
Much information can be extracted from the temperature- and angular-fluxuations of the signal.