CAPTCHAs are those little boxes of distorted text that you need to type out in order to register for many Internet services.
Basically, CAPTCHAs attempt to tell if someone on a Web site is human or a computer. Spammers have been known to send automated comments to blogs and guest-books in an attempt to raise a site's PageRank.
Google is experimenting with audio CAPTCHAs for users with visual impairments. In September of 2009, Google acquired reCAPTCHA, a company that supplies CAPTCHAs.