Captcha is a type of Challenge-Response that is used on websites
to ensure that they are only used by "real" people, and no automated programs.
This can be used to prevent automated access to the website, or to prevent Spam
programs from sending emails from the website's Email Form.
Here's some extra reading...
Wikipedia's entry for "Captcha"
The W3C organisation's opinion about Accessibility problems with "Captcha"
Captcha unacceptably discriminates against people with many disabilities.
Captcha is very restricted as an anti-Spam measure,
as it plays no role once Spammers already have your email address.
Don't use Captcha for Email Forms.
Instead, use other anti-Spam measures such as trainable Spam Filters or Whitelists.
Only use Captcha for Website access if you already know who your individual visitors are,
and you're sure nobody will be unable to use it.
Instead, use private logons with secure passwords.