Copyright: Open Licenses & Public Domain

"A work of authorship is in the “public domain” if it is no longer under copyright protection or if it failed to meet the requirements for copyright protection. Works in the public domain may be used freely without the permission of the former copyright owner. " - U.S. Copyright Office  There are four common ways that works arrive in the public domain:

• the copyright has expired (usually after the life of author + 70 years) 
• the copyright owner failed to follow copyright renewal rules (specific to works created after 1922, but before 1964)
• the copyright owner deliberately places it in the public domain, known as “dedication,” or
• copyright law does not protect this type of work.

CC BY	Content can be remixed, reused, even sold commercially but attribution to the original source author must be provided.  This is the most open Creative Commons license.
CC BY-SA	Content can be remixed, reused, even sold commercially as long as the original material is attributed to the original author and any derivative works are shared alike with the same license going forward.
CC BY-ND	Content can be reused both commercially and non-commercially but cannot be remixed or changed in any way.  Attribution must be given to the original author.
CC BY-NC	Content can be remixed, reused, and distributed non-commercially with attribution to the original author.  Derivative works do not have to be licensed under the same terms.
CC BY-NC-SA	Content can be remixed, reused, and distributed non-commercially with attribution to the original author.  Derivative works must be licensed under the same terms.
CC BY-NC-ND	Content can be reused non-commercially but cannot be remixed or changed in any way.  Attribution must be given to the original author.  This is the most restrictive Creative Commons license.

More on the Creative Commons License can be found here.   

Images licensed by Creative Commons under CC BY 4.0.