CAPTCHA is a security test to determine whether or not a computer user is human. This will ensure that only human visitors will actually submit web forms, and not spam-bots. Spam-bots brows the internet and go from website to website and submit spam everywhere they can. Programmers can create automated computer programs to accomplish many tasks, both for good as well as for bad. Some of the "bad" includes spam-bots that can visit a large number of websites following links from page to page and collecting emails listed on these web pages, submitting contact forms, or even trying to break passwords by attempting to login over and over again with computer generated passwords.
Fortunately, it is not yet possible for a non-human ( ie: a computer program ) to extract a word or a combination of letters and numbers from an image, if the letters and numbers are not all of the same size and of similar font. This is why we use the CAPTCHA Image Verification security method to ensure that only people actually submit our web forms.
FYI: CAPTCHA is an acronym for "Completely Automated Public Turing test to tell Computers and Humans Apart."