How does Google Trends work?
By analyzing a portion of Google web searches Google Trends computes how many searches have been done for the entered search terms, relative to the total number of searches done on Google over time. Google then shows in graph format, the results – Called the Search Volume Index Graph.
Located under the Search Volume Index graph is the News Reference Volume Graph. This indicates the number of times a topic appeared in Google News stories. When Google Trends detects a spike in the volume of news stories for a particular search term, it labels the graph and displays the headline of an automatically selected Google News story written near the time of that spike.
Under the search and news volume graphs, Trends displays the top regions, cities, and languages in which people searched for the first search term entered.
According to Google "The data Trends produces may contain inaccuracies for a number of reasons, including data-sampling issues and a variety of approximations that are used to compute results. We hope you find this service interesting and entertaining, but you probably wouldn’t want to write your Ph.D. dissertation based on the information provided by Trends. The information provided by Trends is updated daily, and Hot Searches is updated hourly."