A knurled nut is a common type nut used when a nut will have to be hand tightened or loosened frequently. 'Knurl', in this case, refers to a very small bump, and a knurled nut will have dozens of tiny bumps or ridges along its outer surface, intended to give your fingers traction and make it easier to make fine adjustments without a wrench.
While there are no formal guidelines, as a matter of practice knurled nuts tend to have tiny bumps, and thumb nuts tend to have a series of vertical ridges, presumably because if you are adjusting something with your thumb you are likely to be making unidirectional, gross adjustments, rather than fine tuned back-and-forth twiddles. Plier wrenches, calipers, and compasses are likely to have knurled nuts for tightening.
A long knurled nut is sometimes called a knurled spigot. Those applications requiring more force but still needing hand-tightening may prefer a wingnut.
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