Boolean Logic allows you to combine keywords to make a search more precise.
AND - Narrows a search.
example: airplanes AND weather -- will retrieve all articles with both airplanes and weather in the article.
OR - Expands a search.
example: airplanes OR pilots -- will retrieve all articles with just airplanes, all articles with just pilots, and all articles with both airplanes and pilots.
NOT - Removes a term from a search.
example: aviation NOT aerospace -- will retrieve all articles about all topics on aviation but remove all articles with any mention of aerospace. Use NOT sparingly because you often lose many useful pieces of information.