Reference sources provide a broad or general introduction to a topic, and often provide links or "pointers" (e.g., bibliographies, references, authors) to other information sources. They are a great place to begin exploring your research topic, and may help you convert a very broad topic into a more focused, more specific research question. Specific reference sources include dictionaries and encyclopedias.
"In a nutshell: You should not use Wikipedia by itself for primary research (unless you are writing a paper about Wikipedia)."
--from Wikipedia article: Researching with Wikipedia