When editing your site it is important to realise that there are actually two copies of your site. There is of course the published public version of your site, but there is also the one being edited which is within the CMS. As you edit your site your are encouraged to 'save' your changes every five minutes or so. This saves your edits to the local version of your site (within the CMS). Of course regular saving is important so that you don't lose any of your edits due to internet outage or computer crashes.
However saving changes does not make those changes visible on your live page (whether your test site or public page). To make your changes permanant and visible on the main version of your site click 'Publish'. It is only necessary to 'Publish' at the end of your session, even if you are editing several different pages. Note though that if you preview one of your edited pages and then click a menu or other link you will have then left the preview area and be back on the public version of your site - this meaning you will now be viewing the unedited version of your site.
On a big website, publishing changes is not very fast, so you should only do this at the very end of the day when you are happy with all your changes.
Some changes are live automatically. Typically changes to products and zoomed gallery pages. The pages that require publishing are your single design pages and index pages and home page.
You cannot publish only 1 page - ie when you click "publish" your entire site, wether edited ot not is published again.
You cannot undo a publish