What is reCAPTCHA?
reCAPTCHA is a free service that uses distorted images, words, and shapes to tell whether a web user is a human or a computer. You've probably seen them — distorted text at the bottom of Web registration forms. This idea has a very interesting history. Carnegie Mellon University developed CAPTCHA to prevent a piece of malicious code from submit fraudulent transactions. It did this by displaying distorted letters that only a human could decipher and key in.

With reCAPTCHA, all of these human key strokes now help digitize books. So, when a book is scanned, the software tries to decipher letters and digitally copy it. Occasionally the software cannot recognize the letters, particularly with really old books that have smudged print. So reCAPTCHA was developed. reCAPTCHA  presents you with two challenge images. One is the standard CAPTCHA that make sure you are a person, while the second word is a genuine scanned image from a book. When you enter what you see, your response is recorded, instructing the digitizing software what that image is.

Why are the images sometimes random images that are not words or standard letters?
Most of the time the images look make sense to a reasonable person. However, occasionally mathematical symbols, Greek equations, or even indiscernible random images appear. This is due to the fact that the software cannot recognize what the image is. It doesn't happen often, but when it does, it can be confusing. Luckily, however. one must simply click the submit button for a new challenge word. reCAPTCHA will refresh, and you should be fine.