What is a non-public gallery?

A non-public gallery is one that is not listed in a user's gallery list.
Unlike a password protected gallery, users do not need a password to view this gallery, they just need to know the URL for the gallery. This is a useful tool if you do not want to have to set a password for a particular gallery and give it out to specific people; you can simply give them the URL of the gallery or image.

Who can view a non-public gallery?

Anyone is knows the URL for a non public gallery can view the gallery. Viewers do not need a password to view a non-public gallery, but a non-public gallery can be password protected.

Can a non-public gallery be password protected?

Yes, a non-public gallery can also be password protected. You can add a passkey to a non-public gallery the same way you add one to all other galleries.

Can a non-public image be in a public gallery?

Yes. A non-public image can be in a public gallery. When editing an image, you can decide whether an image is public by checking or unchecking the box next to "this image is publicly viewable". Even in a public gallery, if you uncheck this box, the image itself will be non-public and will only show up to the image owner.

Are subgalleries of non-public galleries non-public?

No. Subgalleries of non-public galleries are not non-public unless you manually set them to be.

How are password protected and non-public galleries different?

A password protected gallery will show up in your gallery list. Viewers will not be able to access the gallery unless they have the passkey.
A non-public gallery will not show up on your gallery list. Viewers will need to know the URL of a non-public gallery or image in order to view it.
A gallery can be made both non-public and password protected. Viewers will need to know both the URL of the gallery and the password for the gallery in order to view it.