Captcha is a relatively new device used to verify that you are a human rather than a web robot, or 'bot' by making you pass the Turing test before accepting the form.
A bot can fill in a user name and password on a sign in form a thousand times a minute until eventually they get a result. A website has no idea of telling who or what is filling in the form, it is simply looking for the correct the combination of user name and password it will let the user in regardless of who completed the form.
Q) Why does a bot keep filling in our contact form with a stack of links to a Viagra site (a growing trend), after all, the email just comes back to one person in our office and is hardly reaching the masses?
A) The bot actually includes a BCC list of recipients and sends out mail to thousands of people, you get one copy and are unaware of the BCC copies that went out using your email address and your mail server.
A bot can easily read text on html pages and copying any email addresses and other information is its aim.
The bot scans all pages and looks for forms with input fields like username and password, the bot puts some text in and clicks the button until it gets the correct combination and then reports the information back to head office.
Hence captcha was created. In defence of bots filling in forms over and over we display a CAPTCHA image on screen and if you can't type into another box what you see then your are regarded as being a bot and any forms sent without the correct answer will be ignored.
The image cannot be too obscure as we must allow for all walks of life.
The problem is that the bots got smarter and can now read basic images, in an attempt to to make it harder for the bots to read the words the images are getting more obscure, to the point where even a human has trouble reading them.
If a bot can't work out what the image is it reports it back to a low paid worker in some distant country that assists the bot on figuring it out, once the pattern is worked out the human gets $2 and the robot discards the human as they are no longer required (charming).
Using on the fly animated images with changing styles makes it very difficult for a bot to read even a short 4 digit image, but is clear to a human.
* Note: This is the only code on this entire site we did not write from scratch and would like to honour László, a talented programmer from Hungary who conceived and implemented this top technology.
I hope you agree that László's idea is both rocket science and a delightful experience compared to some.