When used with caution, the open web can be a useful source of definitions and keywords --
Think of words or phrases that best describe the topic you are searching. It is important to remember that individual authors might use different language to describe a topic. Concepts can be identified using various words so it is helpful to come up with synonyms or related terms, as well.
The drawback of using natural language or keywords is that the entire bibliographic record is searched. Because of this results will vary and may be unrelated to the topic.
Subject headings or controlled vocabulary are terms assigned in the database to identify particular topics. Database thesauri can be used to identify subject terms.
Subject terms search the subject heading field of the bibliographic record. This specificity limits the search and creates more relevant results.