Keyword searching is generally what you use when you begin a search. Keywords will be the main ideas in your research question or topic--typically the nouns--people, places, ideas, things.
Keywords have a profound impact on search results. Using the right words will speed up the research process; choosing the wrong ones can bring the search to a painfully screeching halt.
It helps, too, to have synonyms for your keywords. There's usually multiple terms you can use for a concept.
Take, for example, the use of Ritalin in adolescents to improve grades.
How does one think of synonyms? Run your search first. Look at your good results. Generally within your good results, you'll see other terms being used that you can pull into your search. OR...run a Google search on your words. Or run a search of your words in a Thesaurus. Or use the keyword generator tutorial.
You may also want to think of narrower terms or broader terms, too, depending upon how many results your search is generating.
Another way to think of keywords is to use mind maps.