Search terms--or keywords--are important words/concepts found in your research question or thesis--you'll use these keywords to find information on your topic.
A quick and dirty way to pull keywords from a research question/thesis is to choose the most important nouns; most other words are irrelevant--particularly articles, and words such as effect or impact.
Using keywords to search will always retrieve more results than phrases or sentences.
Think of synonyms, too--synonyms will always expand the number of results you retrieve (see Search Strategies).
Image source: Producer. CC BY-SA 3.0. Wikimedia Commons
Keywords have a profound impact on search results. Using the right words will speed up the research process, while the wrong ones can bring to it to a painfully screeching halt.
If the keywords you initially choose do not give good results, try others on your list, try search strategies, or ask a librarian for help.
Use the chart above to document keywords related to your topic. Keep it by your side when you start your research.
Image source: Powernowllc. CC0 1.0. Wikimedia Commons.