Google Trends works by analyzing a portion of Google searches to compute how many searches have been done for the terms entered, relative to the total number of searches done on Google over the same time. Although the data provided by Google Trends is updated daily, Google includes a disclaimer that 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."
It's possible to query up to five words or topics simultaneously in Google Trends. Results are displayed in a graph that Google calls a "Search Volume Index" graph. Data in the graph can be exported into a .csv file, which can be opened in Excel and other spreadsheet applications.
A feature called "Hot Searches" displays a list of the day's top 40 search queries in the United States. Another feature called Google Trends for Websites analyzes website traffic, rather than traffic for specific search terms. The data includes unique visitors and a regions column, which shows the percentage of visitors from a specific geographical region. "Also visited" and "also searched for" columns show other websites and other search terms that a site's visitors are likely to visit and search for.
In 2008, Google announced Google Insights for Search, a service that offers more advanced features for displaying search trends data. Search engine marketers can use Google Insights to fine tune their keyword strategies because it allows them to examine search cycles and patterns for specific keywords and a keyword's popularity by location and time range.