I was looking for some images on Google Image and finally clicked on Google Image Labeler that is on the google image page to see what that was all about.
In my opinion Google is using it's users with Google Image Labeler to give better descriptions for the bazillions of images that they have indexed. They use the game and point method and I played it for about 30 minutes (yes, I was hooked for a little). I was teamed up with a random person and we both are presented with the same image and then have to type descriptions for the image and can't use certain words that have been used (labeled in red). Someone asked me why the random person and I responded so that you can't label a picture of rose as "BOTTLE" with a friend.
Points are awarded for descriptions that you both match on. You have a certain time limit to accomplish this and you can enter words, or pass. I got almost 3000 points with one person and averaged 700 with another. It helps to be teamed up with a fast typist because you want to go through as many images as possible in your time limit.
Google keeps track of a ladder board and all your points or you can remain anonymous (guest). Try it out...