AdBlock blocks ads, Ghostery blocks "trackers", and NoScript blocks all JavaScript (and a few other dangerous potentials). NoScript doesn't necessarily block ads but it does block "trackers" by default.

I used Ghostery for a while but found it redundant and onerous since I also run NoScript. I agree with the parent that said Ghostery was a poor man's NoScript, though Ghostery only blocks known trackers (analytics tools) and will let random evil scripts through. NoScript allows you to whitelist script sources by domain, so you usually see "Enable example.com; Enable analytics.google.com; Enable quantcast.com;", etc., and you can just enable example.com if you don't like the trackers.

NoScript is really important and useful I think. Less convinced on Ghostery. AdBlock is useful but I don't know if I'd call it "important" really; it provides a great visual convenience to be able to read sites without being bombarded by ads, but one can live safely without it if necessary.