In an academic context, a license is permission you get from the copyright owner of the work you want to use. A license basically grants permissions, but sometimes it states restrictions as well. It specifies what can and cannot be done with a work.
An OPEN license is a type of license that grants permission to access, re-use and redistribute a work for free, with few or no restrictions. With open licenses, creators still maintain the rights to their copyrighted work -- they are not "giving" away their work or their copyright.
Bottom line? Think of an "open license" as "free + permissions."
If you see an open license, that's how you know it's OER! See below for common types of open licenses.
Text source: Adapted from "Learn OER" modules, Open Washington, by the Washington State Board for Community and Technical Colleges, licensed under CC BY 4.0. Image source: Open books graphic by opensource.com, CC BY SA 2.0
"Creative Commons" licenses are referred to as "CC" licenses, and they are examples of open licenses. So if someone creates OER and wants to share it with others, then they put CC licenses on their work to make it clear that they are sharing their work.
A public domain work is a creative work that is not protected by copyright, which means it’s free for you to use without permission.
Works in the public domain are those whose intellectual property rights have expired, have been forfeited, or are inapplicable.
There is an additional CC license, called the "CC 0" (CC Zero) license that releases modern works into the public domain with a Creative Commons license.
This enables works that are no longer restricted by copyright to be marked as such in a standard and simple way, making them easily discoverable and available to others. Many cultural heritage institutions including museums, libraries and other curators are knowledgeable about the copyright status of paintings, books and manuscripts, photographs and other works in their collections, many of which are old and no longer under copyright.
Source: "Difference between open license, public domain and all rights reserved copyright" by Boyoung Chae is licensed under CC BY 4.0