reCAPTCHA is a free security service that protects your websites from spam and abuse. It is designed to distinguish between actual human computer users and malicious bots. 

reCAPTCHAs were originally decided to identify words that are usually failed to be recognized by the OCR short for optical character recognition. Such words do not match to any words in the dictionary, as OCRs have not deciphered them properly, and are transferred to become CAPTCHAs, where multiple people submit their notion of the word.