1. A computer program product readable by a computing system and encoding a set of computer instructions implementing a method for managing product knowledge comprising a plurality of data items related to products offered for sale by a selling entity, the computing system including a memory arrangement and at least one processing unit, the method comprising: defining a data model of data categories, the data model establishing relationships between data categories; receiving in the computer system one or more particular data items corresponding to one or more of the data categories; receiving user-defined relationship information for the particular data item, the relationship information relating the particular data item to one or more other data items; and presenting the product knowledge, including information about the particular data item, to a user of the system in a manner established by the data model and the user-defined relationship; wherein the data model is constructed from one or more data instance items interconnected using the user-defined relationship items for each data instance item.