Making Your Site Public
Private Preview vs. live
The Site Visibility switch, in more depth than the Getting Started overview.
Where the switch lives
Go to Configuration -> SEO & Sitemap and use the Site Visibility control. While set to Private Preview, only a signed-in admin can see anything on the site, no matter how many pages are published -- everyone else sees a simple check-back-soon page.
Switching back to private
You can switch back to Private Preview at any time the same way -- useful if you need to make a significant change and would rather visitors not see the site mid-edit. Nothing about your pages or content is affected by toggling this either direction.