Subject headings used in the Simmons Library catalog are set headings provided by the Library of Congress. The same subject headings can be applied to subject searches in other library catalogs as they are universal. Your terms will only be searched in the subject fields of the records. Your results are likely to have a higher degree of relevance than in a keyword search. In the Simmons catalog keyword searches are listed in order of relevance using a five star rating system - five stars being most relevant and one star being least relevant.
Many databases use subject headings that are unique to that particular database.
For example Medline uses MeSH headings - medical subject headings. These are highly specific subject terms that are assigned to each article based on the article's content. Scientific and medical terminology may have many synonyms and keywords may not always accurately reflect the content of the article. To assist the user, some databases provide a look-up thesaurus function where you can search the term you want to use and it will tell you the appropriate subject heading to use. When no thesaurus is available you can resort to a keyword search which accepts natural language words and phrases.