Unlisted vs Deactivated vs Password Protected
Unlisted means that the page is not seen in the site navigation and does not appear in search results on your site. But the page is still active! So anyone with the specific URL can still access it. And if the URL is posted anywhere (such as on another site or in social media) it could get added to a search engine index, therefore making it publicly accessible. Unlisted pages are often used as landing pages for targeted marketing, such as Facebook ads with a unique ticket offer. It's a good way to measure the success of an ad campaign.
Deactivated means the page is only visible in Spark. The public URL is not accessible. Anyone attempting to use that URL will see a 404 error page from your site ("Our apologies...")
Password Protected means that a page is only accessible if the user can enter the correct password on the landing page. This feature is found in the Tools panel (see User Access) on the far right side of dynamic pages. Only one password can be set per page, and it is not available on event or business pages. Use this feature to control access to exclusive information such as special forms, media packets, premium services, employee-only content, etc. Password Protected pages can also be Unlisted. Read more... Password Protected Pages
NOTE: If you have a deactivated URL which is likely already indexed by the search engines, yet you don't want anyone getting the 404 message, you can add a Redirect to point that URL to another page, such as your homepage. Read more... Site Settings - Redirects