Many databases have a thesaurus or controlled vocabulary such as MeSH (Medical Subject Headings), including APA PyscInfo, Medline and CINAHL . They list standardized subject headings used by database indexers to describe what a source (article, book or conference paper) is about, enabling citations to be organized and found by users. Each source within a database is designated a MeSH term(s) according to the subject(s) it covers. Therefore, when selecting a MeSH term you will retrieve all records for which that subject has been designated. A MeSH search is more advanced than using your own keywords as it will retrieve results that focus on the subject you've selected.
Searching using MeSH terms allows for terms to be searched as a Major Concept (the search will return only records for which the subject heading is a major point of the article) and also provides the ability to Explode (expands the subject heading to retrieve all references indexed to that term, plus any narrower terms that are connected). For example, articles which talk about cancer, tumours or leukaemia will be mapped to the Medical Subject Heading [MESH] term Neoplasms. A worked example of MeSH in action is demonstrated by PubMed.