After editing, any published node needs administrator's approval like comments. How can I do this with Drupal 6?
Unfortunately, Moderate is Drupal 5 only. Another alternative is to use the built in revisioning system. Set it up so that any edits create a new revision that does not get published (this is in the settings for the content type). Set the
publish revisions permission so that only administrators (or whatever your admin user role is) can publish a newly created revision.
You might want to follow that up with an view that shows recent unpublished revisions so that an admin can see the most recent page changes.
Using Oswald's answer, and content that gets edited will become unpublished, and therefore not be visible by users who cannot view unpublished content. With this method a user will be able to carry on viewing the unedited post until the new revision is published.
Check this page for more info on revisioning in Drupal.