When editing a published page, you'll be able to save your work as a draft by clicking Save Draft at the upper right. There can only be one draft per page, which means it is shared. Any webmasters working on the site will see the draft.
Pages saved as drafts are indicated in the site map as pictured here:
Site visitors will continue to see the last published version.
To cancel a draft, highlight the page in the site map and click the Cancel Draft button.
To work on a draft, just click the page in site map and click the Edit button. You'll always be taken to the draft.
To see the live version of the page, just open up the regular website.