The Google “search by image” option allows you to perform reverse search on an image and discover all sorts of content that's related to that specific image. I have used this feature a couple of times to successfully find the source, author or photographer of certain images that I found on the Internet.
You can search the web by images by going to images.google.com, click the camera icon in the search box and enter an image URL for an image hosted on the web or upload an image from your computer. To make it even easier to search with images you encounter on the web, the Google Chrome team have also released extensions for Chrome and Firefox that allows users to search with any image on the web, simply by right-clicking on the image.
A recent update on the Chrome Beta channel has done away with the need for this extension.
Google Chrome 30 Beta now includes search by image in the UI of the browser. Simply right-click on an image and you will see the option to search for it within your default search provider i.e. Google.