Content types are a powerful new enhancement introduced in WSS 3.0. The central idea is that a content type defines the underlying schema for either an item in a list or a document in a document library. However, it’s important to understand that content types are defined independently outside the scope of any list or document library. After you have created a content type, you can use it across several different lists or several different document libraries.
Unlike a list, it is not possible to create a column in a content type directly based on an underlying field type. Columns within a content type must be defined in terms of existing site columns.