Delete Page

This completely deletes (and un-publishes) the page from your system. It will also delete any sub-menus under that page. No trace of the page will remain in the CMS. Any links to the page will fail (404).

Disable Page

This turns the page off - your CMS system will ignore it going forward. It will not be included in menus, and it will not publish. However, the page remains in your CMS tools, so that you can re-enable it later if you need to.
This does not automatically un-publish the page, so you should do that first if you also want links to the page to fail (404) and/or not show up in search engines.

Archive Page

This tells the system that you are not working on this page anymore. It disappears from your pages in your CMS tools, which helps to reduce clutter. You can find it under the "Archives" link in the Website Manager if you need to.
However, the page still exists in your website and continues to works normally. You do this because you don't want old links to break, and you want to keep the SEO that the page has earned.

This does not automatically remove the page from menus and site maps; to do that, you should also set the page visibility (located under configure page) to something appropriate such as hidden.