Summaries of facts, definitions, histories, statistics, and other types of information on large subject areas, organized for quick lookup.
Reference sources are generally the place to begin your research, especially when you're starting out with an unfamiliar field. But they're also where you return when you need to look up formulas, facts, definitions, and other standard details; they tend to pack a lot of information into simple, easy-to-use packages.
Many reference works are available online and are accessible through links from the Library Catalog and from subject or course guides, but many valuable reference resources are still available only in print, and a few highly specialized tools are on microform or CD. Because print-only reference books are in high demand, they are kept in separate, non-circulating reference collections in most UCLA libraries.
Reference sources are rarely peer-reviewed. In fact, because they mostly contain established, factual information, they're sometimes not even cited in academic works, unless directly quoted. Check your style manual for best guidelines.
As compilations of existing information, reference works are decisively in the category of secondary sources. to the point that some people call them tertiary sources.