Google Trends allows you to compare two or more search terms to see their relative popularity and seasonality/trending over time. If you enter the terms into the search bar and separate them with commas, you’ll see the requested terms’ trend history depicted in different colors on a graph spread over a certain time period. You can modify the results by changing the time period and/or region.
Google Trends sample output
With Google Trends, users can also see Google’s estimate of which cities, regions, and languages performed the largest number of searches for a particular keyword Experienced marketers often feel that this data is imprecise (and occasionally inaccurate) because more accurate data from analytics and search advertising campaigns have often
contradicted the results. However, it can give you a basic sense of where your target population is located.
Lastly, plotted on each graph are a few articles/search results related to your keyword query, which correlate to peaks and valleys in the historical search popularity.
Where it gets its data
It gets its data from searches performed on Google.
How it is useful
Google Trends is a great, easy tool for comparing keywords and identifying which are more popular; in addition, you can examine this data over many years with seasonality factored in. Although Google Trends doesn’t supply figures, the graphs are simple to understand and provide a perfect visual of search trends over a particular period of time.
Note that this works only with relatively popular terms, not with long-tail search terms.
It is free to use.
This post contains the content of book The_Art_of_SEO_2nd_Edition