Describing sets is a matter of making a more or less formal description of what is included and what is not included. Many of us do this daily when communicating with others about various ideas, actions, objects. When we make a purchase, we need to be able to tell the cashier what is included in the sale and what is not. When we write a letter or have a discussion on the phone, we need to clearly identify what it is we're talking about. When making rules or defining corrective action, we need to be able to identify what merits the correction and what does not.