From Wikipedia: Reverse Image Search is a content-based image retrieval CBIR) query technique that involves providing the CBIR system with a sample image that it will then base its search upon. Reverse image search allows users to discover content that is related to a specific sample image, popularity of an image, and discover manipulated versions and derivative works.
Super-simplified: Find matches based on a sample image.
TinEye and Google
The primary Reverse Image Search engine that I use is TinEye- it gives better results and has additional features.
Why would anyone need this?
A reverse image search almost seems useless- if you already have an image why would you need another? A Reverse Image Search can link you to the original source or additional info, find high-res sources, and even find images that have been manipulated.
Getting Additional Info
Lets discover who these four old geezers are through a reverse image search:
We get 22 matching results, and the 5th result has the names in the URL: Henry Ford, Thomas Edison, Warren G. Harding, Harvey Firestone. Also note that all of the results are higher resolution than my source.
Using a Manipulated or Cropped Source
TinEye actually does a very good job at finding a match, even if the source is cropped and manipulated.
Notice the waves and horizon are a completely different color.
The Google Comparison:
Expect Google to buy out TinEye any day now.
Search by Color!
If you are looking for that perfect image that matches your color palette, TinEye has a new feature that allows you to search images by color.