CAPTCHA helps distinguish humans from automated web robots to prevent, for example, making unlimited free email accounts for spamming purposes. CAPTCHA typically presents a distorted word that the user needs to enter. I hate that we need CAPTCHA but I understand its need and reluctantly make all of you solve a CAPTCHA when you leave a comment on this blog that dramatically helped decrease the amount of comment spam.
reCAPTCHA gives two words for the user to type in, one to test the user and the other from some corpus to help determine a word from a scanned image. When I purchase from Ticketmaster I had to deal with reCAPTCHA which helped understand words from old New York Times articles.
I'm less a fan of reCAPTCHA, forced to perform a task for someone else even if that task is admirable. Google will use reCAPTCHA to help in digitizing scanned images from their books project. Google makes revenue from this corpus and I don't see why I should be forced to perform tasks for them even if it takes only a little bit of my time.
I assume Google will replace all the CAPTCHAs on their properties with reCAPTCHA, including Blogger, the software that powers this weblog. So if you want to leave an anonymous comment on this blog without signing into Google, you will have to give Google some of your time.