Pius Library owns many subject-specific encyclopedias, which can be an excellent place to start your research and get background information about your topic. You can use the Library Catalog to do a keyword search for your subject AND "encyclopedia." For example, you might want to search for "religion AND encyclopedia" or "psychology AND encyclopedia." This technique also works for dictionaries, handbooks, etc.
Gale Virtual Reference Library is a collection of electronic encyclopedias and other reference books; it can be a good place to find background information on a wide range of subjects.
If you need help finding an appropriate encyclopedia, just ask a librarian!