Open Educational Resources are teaching, learning and research materials in any medium – digital or otherwise – that reside in the public domain or have been released under an open license that permits no-cost access, use, adaptation and redistribution by others with no or limited restrictions.
Source: The William & Flora Hewlett Foundation
The terms "open content" and "open educational resources" describe any copyrightable work (traditionally excluding software, which is described by other terms like "open source") that is either (1) in the public domain or (2) licensed in a manner that provides users with free and perpetual permission to engage in the 5R activities:
The term “public domain” refers to creative materials that are not protected by intellectual property laws such as copyright, trademark, or patent laws. The public owns these works, not an individual author or artist. Anyone can use a public domain work without obtaining permission, but no one can ever own it.
There are four common ways that works arrive in the public domain:
Basically, most things created before 1923 usually qualify.
Many resources such as OERs are made and intentionally licensed as open. Usually this is using the Creative Commons License.
The goal of using OERs is to lower the textbook/materials cost for students. OERs don't have to totally replace the textbook but they can.