Captcha protects your website from unwanted users such as automated machines, bots and spammers. Once you have decided to use Captcha for your site, there are some important criterions you must take into consideration:

Accessibility
Captcha must be accessible for every user – for the normal and the blinds.
That means that the captcha image shouldn’t be too small and it should offer an alternative (audio one for example) for the ones who can’t see well or don’t see at all.

Image security / complexity
Captcha images should not be easy to percept.
They should be vague and distorted so no bot can solve it – only humans.

Script security
A Captcha code should consist of images that are not readable by bots as well as questions that can be answered only by using human intelligence.

Security of CAPTCHA wide spread use

When Captcha system is adopted by a great number of sites it could weaken its security effect, since a parser could make it easy for bots to solve the Captcha test. Since it is highly important that Captcha will be of strong security even when it is widely used, it is recommended to use well-tested implementations of *reCAPTCHA than to create your own CAPTCHA script.

* reCAPTCHA is a free service that helps to digitize old media such as books, newspapers and radio shows so they can be used as CAPTCHAs