Copyright, according to the U.S Copyright Office, is "a form of protection grounded in the U.S. Constitution and granted by law for original works of authorship fixed in a tangible medium of expression." Basically, it is law that determines who has the right to display and use creative works. Permission from the original creator is usually required in order to display or reuse a creative work (such as a photograph or image) that is under copyright.
Creative Commons (CC) is a form of licensing for creative works that allows people to reuse the work without direct permission from the creator, as long as certain rules are followed. Different forms of CC licenses have different rules; one of the most common is that to use a work you must attribute the original creator - aka cite your sources!
Public domain works are those which are not under copyright, either because the previous copyright has expired, or because the creator of the work explicitly chose to make it public domain. Public domain works are free to reuse and do not require attribution, though you can still provide it if you want to.