According to WIPO: "Copyright (or author’s right) is a legal term used to describe the rights that creators have over their literary and artistic works. Works covered by copyright range from books, music, paintings, sculpture, and films, to computer programs, databases, advertisements, maps, and technical drawings".
A work is in the public domain when it is no longer copyrighted, because the copyright duration or term has expired, the copyright owner has released the work, or the work was never copyrighted.
Works in the public domain can be copied, shared, distributed and translated by any person for free and without needing permission from anyone.
Creative Commons - a nonprofit organization that works "to increase the amount of creativity (cultural, educational, and scientific content) in “the commons” — the body of work that is available to the public for free and legal sharing, use, repurposing, and remixing." (Creative Commons, n.d., para. 1)
Creative Commons licenses provide a way for people to share their work and make it available for others to build on and reuse. It is free to download, adapt, distribute, and transmit without having to ask permission. Depending on the license, however, there may be certain conditions: you may only be able to use the content for educational purposes, you may have to give attribution, etc. Because licenses vary, always be sure to check the exact terms of the license before using an image.
Attribution: others can use the work however they like, so long as they give credit
No Derivative Work: other can copy, display, or perform your work, but it must be verbatim
Non-Commercial: other can use your work, but for non-commercial purposes only
Share Alike: others can distribute derivative works, but only under the same terms as the original license