Many databases have a thesaurus or controlled vocabulary such as MeSH (Medical Subject Headings). 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.
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. Databases that use MeSH terms include APA PsycInfo, Meddline and CINAHL. A worked example of MeSH in action is demonstrated by PubMed.