Classification systems are an easy way to assign objects such as products, services, or even company profiles to a category. Classifying information offers many advantages when dealing with a large amount of different information. For example, it enables one to find information by group or to easily detect similar items. They are meaningful wherever a large set of different, but sometimes similar information is managed. In order to clarify the meaning and the application of classification systems, the following text starting with an example in the area of electronic product catalogs. Important background information about classification systems is given as well as a list of typical classification systems. The text will describe the advantages of classification systems and their meaning for future e-commerce scenarios. The main problem of classification systems today is focused as is the compatibility of classification standards and therefore the reclassification process.