Classification deals with organising and setting definitions. Classification systems are used to break down statistical objects into categories (groups). Each object should only be able to exist in one of the categories of the classification system. A simple classification is the breakdown of man-woman.
A classification system can have a hierarchal structure and have several levels. NACE (The European Classification of Economic Activities) is an example of a complicated classification system with many levels.
A classification system can be connected to one or more variables and can be used for collection and presentation of data.