They allow terms to be combined in different ways to enhance a searching. AND Combining terms with AND will retrieve all records containing both term 1 AND term 2 OR Combining terms with OR will retrieve records containing term 1 or term 2. Indexing is essentially a system of controlled keywords used to describe each record in the database. In this way, variations in the language and terminology used by the authors of articles can be compensated for. A search on this term should retrieve all relevant articles.
Many databases use indexing.
Embase uses Emtree, a similar but less sophisticated thesaurus index. Using complex indexes like MeSH takes some practice but is well worth the effort. An excellent article by Lowe and Barnett covers all of the salient points 2 . Using Truncation Truncation is a facility offered by many databases which enables the searcher to look for several variations of the same word. For example, if searching for articles on hypnosis, the searcher would want to include hypnotise, hypnotise, hypnotised hypnotising, hypnotics, as well as the US spellings hypnotize etc.