What is the difference between a 'hardtail all-mountain' MTB and a 'full suspension all-mountain' MTB?

A full-sus MTB will have both a sus fork and suspension for the rear wheel. A Hard Tail MTB will have a sus fork but will look like a regular bike in the frame, with no suspension for the rear wheel. "All-Mountain means that the bike is light enough and sized well enough to be able to be ridden uphill while still being strong enough to hold together for a decent downhill run.