How is a dichotomous key used?

A dichotomous key is a special document that allows the reader to 'key out' an organism to some level, sometimes species, sometimes a level above species. Basically a key is a series of double statements. Each double statement has only one possible right answer; either the organism has some trait, or it doesn't. Depending on the answer, you are then taken to the next level of statements, each pair of statements whittles down the possible number of species that your organism could possibly be, and eventually lands you on the species, or genus that your organism is in.