For example, if you search on Image Swirl for [washington], you'll see 12 image thumbnails including President Washington, the Washington Monument, a map of Washington D.C. and the Capitol Building:

Once you find the group of images you're interested in, you can click on the thumbnail and a cluster of images will "swirl" into view. For example, here's what you'll see if you click the image of the Washington Monument:

You can then further explore additional sub-groups within any cluster. The interface may look familiar to those of you who have tried Google's Wonder Wheel available in the Search Options panel, which enables you to explore related search queries.
Official Google Blog: Explore images with Google Image Swirl, now in Labs