"Albums set to the most private visibility setting - 'Sign-in required to view' - will not be displayed unless the person viewing is included on an individual album's 'Shared with' list. Your unlisted gallery has an authorization key in the web address; this key is a combination of letters and numbers which makes the web address very difficult to guess," explains Google.
The name of this feature is confusing: while the standard gallery only includes public albums, the unlisted gallery displays both public and unlisted albums. At least you only need a single link to share all your public and unlisted albums.
{ via François. }