"Open Educational Resources are any type of educational materials that are in the public domain or introduced with an open license. The nature of these open materials means that anyone can legally and freely copy, use, adapt and re-share them. OERs range from textbooks to curricula, syllabi, lecture notes, assignments, tests, projects, audio, video and animation." (UNESCO).
Open textbooks are typically freely available under Creative Commons License and available for anyone to use. Users can add their own material or change what exists and re-share. Some are peer reviewed, while others are not.
Open courses are curricula that professors at various universities have made available under Creative Commons License for anyone to use. They may consist of recorded lectures, classroom notes, syllabi, assignments and other supplementary materials. Please note that some of these courses use traditional (and perhaps expensive) textbooks.