A controlled vocabulary is a standardized set of words or phrases used in search tools such as databases and catalogues to aid in document retrieval. In these tools, you often see a controlled vocabulary in the form of 'subject headings' or 'descriptors'. These subject headings are applied to each resource to describe the topic areas of the document.
Using a controlled vocabulary in your searches helps reduce ambiguity inherent in language where the same concept can be given different names and to ensure consistency.
Controlled vocabular helps solve the problem of:
1) Homographs (words that sound alike)
cranes (lifting equipment)
2) Synonyms (words that have the same or similar meaning)
manage, supervise, oversee, administer, etc.
3) Variations in spelling
e.g. American vs British
4) Choice between common and scientific names
e.g peacock or Pavo cristatus
How do you know if the search tool you are using has a controlled vocabulary?
Not every search tool has a controlled vocabulary. If the search tool you are using provides access to a thesaurus, or an option to browse by subject headings or descriptors, then it uses controlled vocabulary.
There are advantages and disadvantages to both methods of searching.
Keyword searching has the advantage of retrieving items with new terms or jargon/slang, but it has the disadvantage of retrieving some irrelevant items as well.
Controlled vocabulary will retrieve all the items indexed under a particular topic, but the disadvantage is that it will miss newer terms and jargon/slang used to describe a particular topic.
Keyword searching can be used in conjunction with controlled vocabulary, so that you are getting the advantages of both methods.
Which method you choose is up to you. Often people will try a keyword search first, and if they find they are not quite getting the results they expect, then they will then look at using the thesaurus to identify the preferred terms for their particular topic.