Open Educational Resources (OERs) can be defined as "learning, teaching and research materials in any format and medium that reside in the public domain or are under copyright that have been released under an open license, that permit no-cost access, re-use, re-purpose, adaptation and redistribution by others" (UNESCO). In higher education, these typically take the form of software tutorials, textbooks, and other instructional materials.
Many educational resources exist online, seemingly "open" because they are on a website or platform that can be accessed by anyone. Having an explicit license included with the materials that specifies exactly what the user is able to do - and not do - with the materials is a key differentiator for an OER. If you place your materials online, adding clear licensing information is a first step to avoiding conflicts and confusion. Creative Commons licenses are a good place to begin.
OERs come in many formats ranging from a tutorials or a textbook to a photograph, a digital sound-bite, or a full curricula. OERs that are well-designed in every way may be useless to you if they're not the right fit your particular needs at that particular time. This framework (Word doc, version 3) can be used to support your OER evaluation process and gauge the suitability of an OER for your purposes.