title: drupal what is vid in node_revision
body: <p>am exploring drupal 6.15 , </p>
  
  <p>am going to do the DB migration ,plain db to drupal db ,</p>
  
  <p>what is vid field in node_revision table in drupal cms ,</p>
  
  <p>i thing vid not an vocabulary id, but plz clear it</p>