The rights granted through copyright law - to the copyright owner - can have an addendum added by the copyright owner only - to give away some of those copy rights.
Using a Creative Commons License or GNU or other legal license addendum to give away some of your legal copy rights, may be referred to as opening your property license (copy right).
Such "Opened Content" is ONLY as opened as the attached license indicates.
After a Creative Commons License is attached to a work, it can not be revoked for that copy of the work...but the copyright owner still retains all rights and privileges of copyright for their work.
Not exactly. Just because content is on the Internet does not mean it is free to be used in any situation. All original content, regardless of access, may have copyright protection, even if it is not declared.
When in doubt, treat everything as under copyright, until proven otherwise.