When adding a new entry you have the option of choosing public, private or unlisted. This setting affects if a listing will be shown when embedded in a page/post. If an entry is public, that entry will shown at all times. If an entry is private, that listing will only be shown when a registered user is logged into your site. By choosing unlisted, that entry will not be shown and is only visible in the admin.
NOTE: When using the shortcode an attribute can be set that will allow you to override the private status of an entry. This is useful if you wish to show a single entry or group of entries regardless if it was set to private or not.