A CAPTCHA is field in a web form, designed as a security feature intended to differentiate between users and bots. Typically a CAPTCHA consist of an image containing letters that are difficult to read. The user must then enter those letters into a text box on the form.
This method is used to put a stop to automated submissions, since it’s quite difficult for non humans to interpret (possibly distorted) pictures and distil the correct information. In short: captchas are pictures showing passwords used to combat automated blog and comment spamming.
The goal is to reduce form spam by making the letters unreadable to bots, but readable to humans.