Have you searched an image on the web? Was it hard to find it?
Well, Google wants to improve the quality of your searches and invite you to play. Yes, play and be useful helping you, others and Google at the same time. You don't need to register an account or do anything else, just start playing.
With Google image labeler you need to label (tag) random images picked by Google and match your labels with another user that use this service and Google chose for you to play with. You have 2 minutes to label random images and match your labels with the user that Google randomly picked for you. You will receive points with any match and more points if the matches are more descriptive for the image.
It's fun and easy way to help others.