“… a carefully selected list of words and phrases, which are used to tag units of information (document or work) so that they may be more easily retrieved by a search…Controlled vocabularies reduce ambiguity inherent in normal human languages where the same concept can be given different names and ensure consistency.” -Wikipedia
Created and edited by a "governing body" to introduce new official terms, phase out old ones, and add new mapping for concepts
Reduces the burden on researchers of a database by mapping human language synonyms to the official term
Searching by a concept or idea, rather than by the presence of a string of letters
Other Controlled Vocabularies
There are thousands of thesauri and controlled vocabularies. Often there are multiple available for one general topic area. The following examples will give you a small idea of the variation and scope of controlled vocabularies.