In the field of information retrieval, a thesaurus is defined as a controlled and structured vocabulary in which concepts are represented by terms. Terms are organised so that relationships between concepts are made explicit. Preferred terms are accompanied by lead-in entries for synonyms or quasi-synonyms (see the relevant international standard). One example of a thesaurus that you might have come across is MeSH, the US National Library of Medicine’s thesaurus of Medical Subject Headings (Figure 8).
The NLM provides a series of webinars and tutorials about MeSH; one that may be of particular interest is Searching Drugs or Chemicals in PubMed. The primary advantage of a thesaurus compared with a non-hierarchical list is that, if the synonyms are well defined, the user can find information without using the exact terminology used by the person who created the database record.