Web forms in online shops, forums and guestbooks are often targets of Spam. You can protect yourself from unsolicited comments by using CAPTCHAs on your website. Along with commercial solutions, you will find many free CAPTCHA services on the Internet, such as captchas.net. Another free and easy to use service is reCAPTCHA.
On http://recaptcha.net you get a free key for using the service on your website. In order to use reCAPTCHA, you need a public/private API key pair. This key pair helps to prevent an attack where somebody hosts a reCAPTCHA on their website, collects answers from their visitors and submits the answers to your site.
To use reCAPTCHA, you only have to add a few lines of code to your website. For detailed instructions on using reCAPTCHA please visit reCAPTCHA’s website.