Wikipedia falls under the category of reference, but also contains no systematic way of checking its facts for accuracy. It is edited by common, everyday people, which makes it an unreliable source. 

However, Wikipedia is still useful because it requires it’s editors to follow a strict set of guidelines for citing its sources. (See resource “Wikipedia: Identifying reliable sources”.)These citations are helpful because they reference areas of information that someone may or may not have been aware of. Think of Wikipedia as a compilation of sources rather than a source itself.