Credo Reference and the Gale Virtual Reference Library are both excellent electronic reference tools.
"Reference materials" typically refer to encyclopedias, dictionaries, bibliographies, atlases, maps and other related works. They can be very useful to the researcher. At the same time, they tend to serve a very specific, limited function in the research process. To cite a few examples:
Source: Bolner, Myrtle S. The Research Process: Books and Beyond. Dubuque, Iowa : Kendall/Hunt Pub. co., 2001.