(No, I am not talking about Google Image Search, although I like that feature. Google Image Search will respond to your Google search with images from the web. I used it to find this great blog that compiles cartoons about libraries and librarians: http://librarycartoons.wordpress.com/.)
Google Search by Image is a new feature that allows you to use an image as your search. You can drag and drop your image if it is online, or type in its URL. If the image is not online, you will need to save the image to your hard drive, and then upload it. Firefox and Chrome add-ons are available to make the process easier. They have a video on how it works here: http://www.google.com/insidesearch/features/images/searchbyimage.html.
Do you have a picture of a building, and need to know what it is? Have a picture of a vehicle, and need its make and model? Give Google search by image a try.