Keyword Searching: Taking your best guess at the terms which will appear in articles on your topic can be an effective search strategy. However, you may get a number of irrelevant results. You also never know if you've found all the articles on your topic.
Descriptor Searching: Descriptors are the controlled vocabulary (or tags) used in a database to organize material. If you can find the right descriptors for your topic, you can focus your search on relevant articles and be sure that you've found everything on your topic.