Classification is a way of seeing the world. In a classification scheme, phenomena of interest are represented in a context of relationships that provide description, explanation, prediction, heuristics, and the generation of ness knowledge. Knowing that information organization isn't equal to knowledge organization, the author first defines the scope of classification and knowledge organization, and then describes the relationship between the classification and the representation and organization of knowledge. At the end, four kinds of classification structures are compared to show their abilities in representing knowledge. In order to utilize these classification structures, it is very important to understand their advantages and disadvantages.