In a weblog the users can apply one or serveral tags to a page, thus indicating the content to belong to a certain category. Then pages can be searched or filtered by these tags. Other objects like messages in a forum or products in a webshop may also be tagged.
A classification is similar to a tag but adds a value, indicating how much or how close the object carries this property. The process is called valuation.
Some webforums feature rankings for users or threads but they mostly offer only one property like 'relevance'. A classification system is more flexible and allows several properties as well as later revison of given values.