A darby is a larger version of a float. Floats are considered an essential tool for plasterers and are also used in finishing. They are used in a similar way to the darby, but you would choose a float for higher precision and a darby for larger areas.
A feather edge is another smoothing tool used in plastering, with a similar method to the darby and float. A feather edge is more likely to be used for lighter tasks, whereas the darby is more heavy-duty.