The fullform of CAPTCHA is, "Completely Automated Public Turing test to tell Computers and Humans Apart." It is a type of security test commonly used on websites in order to determine whether or not the user submitting the form comes from a computer, or a person. There are two main types of captcha systems: text-based and image-based. Image-based captcha systems are typically easier to solve than text-based because they can't be solved with traditional word methods like look up, comparison, subtraction, listing, etc.
Text based captchas work best by using words that includes numbers or letters that can be computed with word method as previously mentioned.