What is Image Verification ( Captcha )?
A security test to determine whether or not the user is human. This ensures that only human visitors will actually submit the form, and not spamming bots that are browsing the web from site to site and submit spam where they can.
Fortunately, it is difficult for a non-human ( ex. 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, the Image Verification security method ensures that only people actually can submit a form.