"Captcha" stands for "Completely Automated Public Turing test to tell Computers and Humans Apart", a term trademarked by Carnegie Mellon University.
The purpose of captchas is to prevent automated log-ins and thus enhance security and reduce spam.
There is a fine line between obfuscating the graphic sufficiently so that it cannot be read by a bot, but not so much as to make it difficult for a human to decipher and enter the code.
A few good captcha examples are:
e-gold login
e-gold calls it a "Turing Number".
OmniPay login
Google Add URL
One that sucks is:
Yahoo Registration
You frequently cannot distinguish upper and lower case in the graphic, and when you fail to read it right, you have to re-enter ALL the sign-up information ALL OVER AGAIN. You can spend an hour until you get it right.
For both a good and a bad example, see the Wikipedia article.
If you want to see some that totally suck, send an e-mail to bob@corp.earthlink.net and try to respond to his autoresponder message which includes a captcha. Frequently, his captchas are completely unreadable, which means you have to send your message again, and try again. I think the purpose of his captcha setup is merely to give the APPEARANCE of accepting non-spam e-mail from the great unwashed (you and me). In actuality, I suspect that he does not want to hear from ANYONE that he has not had his secretary enter into his white list.
Why I believe that is the case is simple - McNamara is an a-hole. He is one reason, among several others, that I discourage people from using Earthlink services. I am now using Verizon DSL and 1and1 hosting and am very happy with both.
