Reverse image search helps you find out where images are published on the web. For example, if you want to see how many places photos of the same car appear on the web, you could use this tool. Or perhaps you want to know if people have been using your copyrighted content without permission. Maybe one of your students downloaded several images to use in a presentation, but forgot to note the websites where they obtained these pictures - Google's Reverse Image Search can help them easily locate these websites.
In this example, I am attempting to find any websites that contain the following picture of a cell that I downloaded to my desktop last month:
From Google Images (google.com/images), I need to click on the camera icon.
Next I can simply upload the image (or type in the URL of the image if I know it).
Google then provides me with a listing of websites that contain my cell image.