When writing content, you work in the 'draft' version. This is not visible to the public until you publish it. You can switch between the published view and the draft view with the buttons towards the top right of the page.
The draft view has a grey background to differentiate it from the published view which has a white background.
Above: The published / draft tabs, highlighted in red
There is actually a third set of content that you cannot see - the archive. Whenever you publish changes, the present 'live' content is moved to the archive, and then the draft content is published - updating the 'live' with your changes.
In the future we intend to have features that let you view previous versions of content and other archive details.