Both types of searches are commonly used together-- a keyword search first to get many results followed by a subject search, to narrow those results:
• Start by doing a keyword search, using words or phrases describing your topic.
• Browse through your results list and choose two or three relevant results.
• Notice the Subject or Descriptor (also called subject headings or controlled vocabulary) field in those relevant records and note the terms used. Keep a list of them to help you remember. (They are also hyperlinked for easy access to all material with that Subject.)
• Redo your search using the subjects or descriptors you identify. Your results will be much more precise than those from your initial keyword search.