I read about this in Wired, and just saw it again. First things
first: a
"captcha" is a picture of a word -
images on Google, entry in wikipedia on
captcha - (usually all messed up: streched, with lines). It is
hard for a computer to read (or use "OCR" on it) but a human can
read it. It is used on websites when you are signing up for an
account (like a new email) and the people who have the websites
want to make sure that you are a human (because sometimes people
make computer programs that sign up for fake accounts [e.g. 1000
emails for spamming or something]). It was invented by some people
from Carnegie Mellon University. They have recently started the reCaptcha program.
The beauty of reCaptcha is that it
doesn't make goofy images: it uses existing ones. From scanned
books. So, everytime someone does this activity, they are actually
helping digitizing the world's libraries. Cool and simple. Wired
article (which actually talks about a variety of interesting
"games" for humans that help computer learn):
http://www.wired.com/techbiz/it/magazine/15-07/ff_humancomp?currentPage=all
Geek note 1: a captcha doesn't actually have to be an image. The
point it is that if you can solve the captcha ["t" stands for test]
then you are an "h": human. So, technically, a captcha can be a
very wide variety of things. This is just the most common. Geek
note 2: reCaptcha uses 1 known word and 1 unknown word. The first
word is to see if you get that one right and the second is to log
for aiding the OCR.