The term special refers to a broad category of various kinds of libraries. The collection, the clientele, or both may be special. For instance, most companies have some kind of library or information center that houses the books, journals, documents, and material relevant to that industry. Historical societies usually have libraries. Museums have libraries geared toward the subject matter of the museum collection. The Folger Shakespeare Library in Washington D.C. is a special library in the sense that it collects books and materials by and about Shakespeare. Insurance companies, law firms, churches, hospitals, oil companies, and banking institutions are other kinds of places where a special library might be located. The Special Libraries Association has 26 different divisions of special librarians in its membership.
You may want to use these special libraries for the kinds of unique materials they make available. For instance, if you are involved with an advertising campaign for a brand of beer, you may wish to use the library collection at the brewery in order to get an understanding of the history of the company and the nature of past campaigns. If the information is not proprietary (that is, held by the company to be private because it involves trade secrets or financial information), you may want to look for clues about the unique brewing process or chemistry that sets that brand of beer apart from its competitors.
Special library collections may or may not be open to the public because corporate secrets are often housed alongside the usual industry or company information. The best rule of thumb is to check the website and call ahead as the hours and visiting policies are likely to differ from collection to collection.