I bet you've used Google image search to find pictures, right? Now instead of using text to find images, you can use an image itself. With Google Images you can either upload a source or provide a link to subject. gImages will give you search results much like Google always have.
This isn't a new idea, but it's great that Google decided to implement it. I'm hoping that Google will have a better chance of finding related images. I already use TinEye, but it only has about 75% success rate.