its a continuously varaible transmission......
basically, its a system that uses pulleys to vary the relationship between engine and transmission. what that does is to continuously vary the effective gear ratio. what that means is that the engine is kept in the prime rev range for the load on the engine at all times. because it doesnt have a torque converter, it is more efficient than a conventional automatic. supposedly, most are a bit weird to get used to, but they are just as efficient as manuals.
as far as multitronic goes, it is audi's cvt. what the multitronic does is pre-select 6 "gear ratios" that it allows the driver to choose from in the same sense that tiptronic allowed you to choose from 5 actual gear ratios.
that good enough? its pretty general, but it sounds like thats what you wanted.