Reference resources are usually encyclopedias, dictionaries, handbooks, manuals and other books that you would refer to when wanting to know a definition of a word or concept, a general overview of a topic, specific information about a certain period of time or event in history.
Background Sources - Materials that provide an overview of a topic, such as core concepts and facts
Exhibit Sources - Materials a writer is interpreting or analyzing
Argument Sources - Information from other authors you are agreeing with, disagreeing with, or building upon