Creative Commons Licenses are designed to giver creators an alternative to the all or nothing nature of copyright. These licenses allow creators to have more granular control over the rights conferred to them by copyright:. Aspects of Creative Commons licenses can be combined in different ways that allow creators to retain more or less control over their creation. There are six types of licenses available and they are listed with basic explanations here.
This license is the least restrictive option, outside of the public domain. All that is required under this license is that you credit the source of the creation when you use it.
This license adds the concept of Share Alike to the basic Attribution license. If you want to use something that has an SA in the license you must share your creation the same way.
Using this license specifically retains a creators right to profit from their creations. You can share their work with attribution as long as you aren't profiting off of it. This license is common academics who want to share Open Educational Resources (OER).
This license combines all the features of the above licenses. This can also be a common license used in OER.
This license starts to put a few more restrictions on what you can do with content. No Derivatives means that you cannot change the content in any way when you share it.
This license is the closest to copyright that we can find using Creative Commons. The creator retains the right to profit from their creations as well as make any adaptations. The only thing users are really able to do is share the content with attribution.
For more detailed descriptions on Creative Commons licensing see their website here.
An important part of copyright that is often overlooked is that it is intended to last for a "limited time". What that limit might be has changed several times over the years. Right now, it means the life of the creator plus 70 years. So on January 1, 2020 all works created in 1924 or earlier passed in to the public domain. Once something has passed in to the public domain there are no restrictions attached to it anymore. This is why we can freely access older journal articles or ebooks on platforms like HathiTrust, Project Gutenberg, or the Internet Archives.
Creative Commons offers two ways to deal with the Public Domain. For items that have passed out of copyright protection and in to the Public Domain you might see the Public Domain Mark used.
Creative Commons also allows creators to license their works with a CC-0 license that immediately lets all possible protections and opens up their creation to be used like other works in the Public Domain. These items to do not require attribution for legal reasons but in academic use you may still want to cite your source for reasons of plagiarism and credibility.