CAPTCHA
Disguised image which people can read but computers cannot
CAPTCHA images are small images, usually of a short string of numbers and letters, blurred or disguised in some way so that while human readers can identify the shapes, a computer cannot decipher the code.
CAPTCHAs are commonly used in registration systems for online services such as webmail and forums, and in blog and news site comment pages, to prevent the automated creation of accounts or posting of comments, both techniques widely practised by spammers on unprotected systems.
Some CAPTCHA systems are known to have been broken by cyber attackers, using advanced optical character recognition.
