“Enter up to five topics and see how often they’ve been searche…
“Enter up to five topics and see how often they’ve been searched on Google over time. Google Trends also shows how frequently your topics have appeared in Google News stories, and in which geographic regions people have searched for them most.”
and
“1. How does Google Trends work?
Google Trends analyzes a portion of Google web searches to compute how many searches have been done for the terms you enter, relative to the total number of searches done on Google over time. We then show you a graph with the results - our Search Volume Index graph. ”
Taken from: http://www.google.com/intl/en/trends/about.html
It tracks the use of the terms as searches.