http://msdn.microsoft.com/library/default.asp?url=/library/en-us/indexsrv/html/ixrefglo_9szd.asp
The catalog is the highest-level unit of organization in Indexing Service. It consists of the content and property indexes and other files that contain the indexed information for the set of files covered by the catalog scopes. By default in Microsoft? Windows? 2000, the "System" catalog exists in the path named "System Volume Information\Catalog.wci", which is hidden when Indexing Service first creates it. The "Web" catalog exists in the path named "Inetpub\Catalog.wci", which is also hidden.