An open textbook is a textbook licensed under an open copyright license and made available online to be freely used by students, teachers, and members of the public. They are available for free as online versions, and as low-cost printed versions, should students or faculty opt for these. Open textbooks are a way to significantly reduce student textbook costs while giving instructors the flexibility to reformat and customize their course material. They are an affordable, flexible alternative to traditionally-published textbooks.
Some open textbooks contain supplemental or ancillary materials (e.g., test banks, quizzes, PowerPoint slides, videos). (BCcampus)
Open Educational Resources
Open Educational Resources (OERs) are a broader category of resources that contains, but are not limited to, open textbooks.
Open Educational Resources are teaching, learning, and research resources that reside in the public domain or have been released under an intellectual property license that permits their free use and repurposing by others. OER include full courses, course materials, modules, textbooks, streaming videos, tests, software, and any other tools, materials, or techniques used to support access to knowledge. (Hewlett Foundation, 2017)
University of Regina librarian Gillian van der Ven has created a guide on Open Educational Resources - visit it to learn more about OERs.