The best way to explain a dichotomous key is to see one.
The chart to the right is a dichotomous key.
The goal is to ask a series of questions with yes or no answers, and continue until you find your organism.
For example in the image to the right. If we see a Duck but are unsure of its name, we can follow the chart.
Does is have feathers? If yes, does it swim? If yes then it is a duck.