Qlikview stores the extracted data from any database or excel files into qvd files. So application by which you make qvd files and store them at a particular location on your syatem are called QVD generators.
You can also do transformation on the data while making qvd files. In order to filter your data or to aggregate it for further usage in reporting...
Hope it will help..
pls let me know in case you need some more information....