How does Google Insights for Search work?

Google Insights for Search analyzes a portion of worldwide Google web searches from all Google domains to compute how many searches have been done for the terms you've entered, relative to the total number of searches done on Google over time. You can choose to see data for select Google properties, including Web search, Images, Product search, and News search (certain properties aren't currently available in all countries/territories).
On ther results page, you'll see:

• a graph with the search volume, indicating interest over time (GMT) for your terms, plotted on a scale from 0 to 100; the totals are indicated next to bars by the search terms (read more about how we scale and normalize the data)
• a breakdown of how the categories are classified
• lists of the top searches and top rising searches
• a world heat map graphically displaying the search volume index with defined regions, cities, and metros