Keys are used to identify different species. A key will usually ask questions based on easily identifiable features of an organism. Dichotomous keys use questions to which there are only two answers. They can be presented as a table of questions, or as a branching tree of questions.
This tree could help you identify a new vertebrate. For example, if it had no fur or feathers and dry skin, you would follow the right-hand pathway at the first and second junctions, but the left-hand pathway at the third junction. This would lead you to identify the animal as a reptile.