Public Domain refers to those media which are not protected by copyright or for which copyright has expired (think of classic books like Jack London's The Call of the Wild as an example). Project Gutenberg, for example, provides access to many of these titles. Creative Commons works still retain some rights, depending on the type of license used: some may permit redistribution, editing, and even commercial use (the most liberal standard, called a CC BY license) as long as the proper credit is given, while others like CC BY-NC or CC BY-ND may not. Creative Commons standards are available here, and a good summary can also be found in the Harvard Law guide listed above.
More than textbooks, but less than a full-fledged curriculum...learning objects can include videos, courses, lesson plans, outlines, and other media to provide a more complete experience. Here are a few useful sites for open-access learning objects:
OER Commons: https://www.oercommons.org/
MIT OpenCourseWare: http://ocw.mit.edu/index.htm
Merlot II: https://www.merlot.org/merlot/index.htm