As a new copy of your page is saved, WordPress will create a “revision” of the previous version.

If you scroll down below the content box when editing a page, you’ll see a metabox (the areas of content below the content box) labeled Page Revisions. In that metabox, you’ll see the various past versions of the page.  By clicking on the time/date for a particular revision, you’ll be able to view the raw HTML content of the page at that time/date.

You can also compare any two revisions against one another by selecting the radio buttons to the left of that time and date, and clicking the Compare button.

To restore a particular time and date, click the Restore link to the far right of the list of page revisions.  Doing so creates a new revision of your page, with the content of the revision you restored, rather than simply deleting all of the revisions since your restored revision.