Captcha are there on most sites to confirm that you are human. The complex tilted, coloured mix of Letters and numbers in various combinations tests the ability of humans to identify them and serves as a good challenge to spam bots while filling forms or posting content online.
This is an acronym for “completely automated public Turing test to tell computers and humans apart” and is a type of challenge-response test used in computing to determine whether or not the user is human. A common type of captcha requires that the user type the letters of a distorted and/or obscured sequence of letters or digits that appears on the screen.
Captchas are used to prevent bots from using various types of computing services. Applications include preventing bots from taking part in online polls, registering for free email accounts (which may then be used to send spam), and, more recently, preventing bot-generated spam by requiring that the (unrecognized) sender successfully pass a captcha test before the email message is delivered. Popular services like ReCaptcha allows anyone to insert Captcha Test challenges on their website and helps to block spam.
Captcha and Blindness
However, Captchas based on reading text or other visual-perception tasks prevent visually impaired users from accessing the protected resource. For non-sighted users (for example blind users), visual captchas present serious problems. Somr new captchas also have an audio recording, which reads out the text, after which the user can type it out, But you still have to see to type.
Because captchas are designed to be unreadable by machines, common assistive technology tools such as screen readers cannot interpret them. Since captchas are often used in initial registration processes (for example eBay and Yahoo!, and some other sites), this challenge can completely block access. Maybe audio recognition can be a solution…
Update: Sometimes difficult CAPTCHA codes can make registration really difficult for users, and it was one of the reasons we needed to switch to an alternative e-mail subscription service for our blog.