You might have noticed that the pages on your site say "state: visible" or "state: published" (this appears to the right of the Add New Item menu, at the top right-hand corner of the main content area of each page).  That is its "publishing state", or how Plone decides how and which people get to see a page.  There are three basic states: private, visible and published.  In more involved sites where different accounts have different levels of permission, this gets more complicated; this page will describe the more typical case where all users have the ability to edit all the pages in the site.

Private - If you designate a page or folder as private, you will need to login in order to see it.  It will not appear in any search results, site maps, or automatically generated menus unless you have logged in.  This is useful for draft content that is not ready for public consumption, and on a larger scale, can be employed to create an intranet for storing internal documents.  Note that this also works for files that you upload, such as, say, board meeting minutes, or strategy documents.

Visible (Public Draft) - This is the state all pages are in until you explicitly change them.  If a page is visible, it can be seen by everyone if they are given a link to it, if it appears in search results, or if they otherwise somehow come across it.  It is most useful for images and pdf files, since you want them to available to people.

Published - Published is Visible, and then some.  Published pages turn up in the sitemap and in the automatically generated navigation that usually appears in the left column (like on this site).  Note that everything appears in the left navigation if you are logged in, but only published pages and folders appear there when you're not.  It's very important to publish all of your web content when you want it to show up in these places.  Additionally, published events and news items will appear in their corresponding sidebars only when published.

You should only change the state after you have saved.  Do not try to change the state while editing.