Save Draft adds a 'Save as Draft' Button to the node_form for content types, allowing the user to click the 'Save as Draft' button to save the node as a draft.
This helps improve usability, as the content creator no longer has to search for the published checkbox to publish or unpublish a node, as they can just click the 'Save as Draft' Button.
If you require more than just a simple way to publish and unpublish node then you may want to check this module out http://drupal.org/project/draft
1. Download, unpack and place in sites/all/modules/
2. Enable the module using Administer -> Site building -> Modules (/admin/build/modules).
3. Now when you create a new node, a 'Save as Draft' button will be added to the form.