OER is any form of educational material (text, video, audio, images, units, entire course, question sets, exam questions, etc.) designed for teaching, learning and research, that is
Licenses are legal tools that allow a copyright owner to give someone else permission to make use of one or more of their copyrights (since copyright is actually a temporary 'bundle' of several rights). Most copyright owners charge fees for these licenses.
Open licenses are a specific kind of license with which a copyright owner can label their works which are available on the internet. These open licenses announce that the copyright owner is giving the public permission up front (without having to ask, or pay any money) to use their work in specific ways.
The open licenses used for OER were created by the Creative Commons non-profit organization:
The list of CC licenses above is modified from the About CC Licenses page of the Creative Commons website, which (except where otherwise noted) is licensed under a Creative Commons Attribution 4.0 International license.