Here's an excerpt from the press release:
Image Search is a tool you can use to find just about any kind of image, but it can sometimes be difficult to find the right image if you can't describe it in words. The new Similar Images feature was developed with just this in mind. Using it you can now find images that look like an existing result simply by clicking on a link. Using visual similarity, you don't have to refine the text of your search, instead, you can just click on the link of an image you like. For example, if you search for [jaguar], you can use the "Similar images" link to quickly narrow your search.