What is the difference between a public, semi-private, and private contest?

When starting a contest, the client can choose between a public contest, a semi-private contest, and a private contest:

Public
A public contest can be viewed by anyone. Both the contest instructions and the submissions are publicly accessible.

Semi-private
In a semi-private contest, visitors need to log in to view the contest. This means that the instructions and submissions are hidden from search engines like Google.

Private
In a private contest, the instructions are visible to logged-in visitors, but the submissions are only visible to the client. Submissions are also hidden from search engines. This option is ideal when discretion is required.