A disk is a gang of rolling, chopping disks, thus the name.
A cultivator is a rack of either spring loaded "shoes", spades or C tines that are pulled down the rows. The cultivator is used to clean up weeds and to loosen the soil around growing plants and in between rows.
A disc or disk gang is for chopping up the stubble after harvest, or to prepare rough soil for further work, in order to plant.
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