Google Trends is a meta-search tool, a search on search. Searches can be interpreted as trends: the more people search for a person, item, etc. the trendier it is (that does not always work, of course, but the general idea should be plausible enough). It works by searching for two or more search terms and comparing searches by plotting them on a time-line. In addition Google Trends plots a second timeline for how often the terms appeared in the news and finds news-stories for peak events in searches.
What makes Google Trends a great teaching tool is not so much the content itself as its potential for spawning interest and engaging discussions in certain topics. When students are confronted with the tool for the first time they, they usually cannot get enough of it and will ask for more. Google Trends can be used in any subject, but I consider it particularly useful for foreign language classes. The tool is a great way of getting students simply to talk.