Hi Arthur,
first of all I start with content repository and storage category. These two terms belong to the content server area and are used in DMS when storing original files to document info records.
The content repository is defined for a content server in transaction OAC0 and can contain several storage categories which are defined in transaction OACT. So this is use to manage the files on the content server better. Also the document class is used from content server side as far as I know. For DMS documents this is DMS_PCD1. The document class is also used for the TREX indexing of files that is mandatory for activating the fulltext search.
Regarding the terms of document type, part, version, etc. please see the online documentation at http://help.sap.com/erp2005_ehp_06/helpdata/en/6e/17b853ff98b44ce10000000a174cb4/content.htm?frameset=/en/6e/17b853ff98b44ce10000000a174cb4/frameset.htm¤t_toc=/en/d1/e8e4535dd4414de10000000a174cb4/plain.htm&node_id=30&show_children=true#jump30
Best regards,
Christoph