The Creative Commons is a source for shared media of all types. The concept is simple: Copyright holders decide which types of uses they allow for their works. Then they release their works into "the commons" with licenses indicating permissible types of uses. Creative Commons licenses combine these terms:
CC - Creative Commons
BY - use requires attribution to the author
ND - no derivative uses
NC - no commercial uses
SA - "Share Alike", or any person who uses the CC work to create a new work must then assign a CC license to the resulting work. Thus, the original work will remain in the commons.
CC0 - the creator has relinquished all rights to the work to the fullest extent possible
Open Educational Resources (OER) are a wide umbrella of online educational media that can be freely used and shared. OER include textbooks, journals, images, video, audio clips, data, software, entire courses, syllabi, and more! Most frequently, they have Creative Commons licenses.