There are two types of revisions. One is Draft and the other is Published.
Draft revisions are copies of the Published/Live page that have been edited by a user. This means if you are completely changing the text on page you can stop half way and save it - therefore not losing any work and not overwriting the current live page.
Published revisions are what your visitors will see. Anyone who visits your website will see the content that appears on your Published page. There is no way that they can see your Draft revisions until you decide to publish them.
