If create new revisions is checked under Publishing options, editing a node will create a revision of that node.  Users that have view revisions or administer nodes permissions will get a Revisions tab on node viewing pages. The Revisions tab interface supports viewing of individual revisions, reverting to a prior revision, and deleting a revision.

Here is an example that illustrates the reverting process:

• If you have revisions {1,2,3,current}, and revert to revision #2, a copy of #2 is made and the copy is set as the current revision.
• After reverting you'll have {1,2,3,4,current}, where current is a clone of #2, and #4 is the previous current revision.

Prior to Drupal 4.7 the node table included body and teaserfields. As of 4.7, these fields are moved into the node_revisions table.