I think there are no situations where the terms are simply synonyms for each other. It is a complicated matter indeed.

Here are simplified definitions:

Public Domain: no restrictions, no copyright claim (not possible in some countries)

Creative Commons: work may be used but in compliance with the stated restrictions

Royalty Free: you buy a license once and can use the work according to the license
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For me, the options of Google image search are misleading. What is "free" supposed to mean? Probably free of charge. But you'd have to investigate each time anyway what the license is and what usage restrictions there are.

[Here][1] is my favorite license.


  [1]: http://www.wtfpl.net