Google Analytics is a free service offered by Google that generates detailed statistics about the visitors to a website.
Google Analytic can track visitors from all referrers, including search engines, display advertising, pay-per-click networks, email marketing and digital collateral such as links within PDF documents.
Integrated with AdWords, users can review online campaigns by tracking landing page quality and conversions (goals). Goals might include sales, lead generation, viewing a specific page, or downloading a particular file. These can also be monetized. By using Google Analytic, marketers can determine which ads are performing, …

How to know what Google Images keywords people are searching for in Analytics?
If the people came using regular search engine, you will see it in Traffic Sources -> Search Engines or Traffic Sources -> Keywords. But if he came from Google image search, he will not be tracked. He will be in Traffic Sources -> Referring Sites as images.google.com. Google analytics has a feature to add custom search engine, so now we will add it.