"Boolify makes it easier for students to understand their web search by illustrating the logic of their search, and by showing them how each change to their search instantly changes their results."
The site uses graphics to show students how using boolean terms like "and, or, not" change their search results. It is very easy to use and I was surprised at some of the results I got by using the boolean terms.
It's free to use. You can also check out this post and video about Boolify.