Open educational resources (OER) are teaching and learning materials that are freely available to students, educators, and the general public. They are openly licensed so that others can adopt or reuse this material for their own purposes. 

To be more specific, when we say that OER are openly licensed, we mean that they can be retained, reused, revised, remixed, and redistributed. In practice this means:

• Retain - the right to make, own, and control copies of the content (e.g., download, duplicate, store, and manage)
• Reuse - the right to use the content in settings like classes, study groups, on websites, in videos, etc.
• Revise - the right to adapt, adjust, modify, or alter the content itself (e.g., translate the content into another language)
• Remix - the right to combine the original or revised content with other material to create something new (e.g., incorporate the content into a mashup)
• Redistribute - the right to share copies of the original content, your revisions, or your remixes with others (e.g., give a copy of the content to a friend)

(See OpenContent.org, UNESCO and OER Commons for the above definitions).

Not all, but many creators of OER choose to apply licenses indicating how others can use their work. Most commonly used are Creative Commons licenses, or a set of statements that indicate the author's wishes when it comes to citing their work, using it for commercial purposes, adapting it, and redistributing it.