Captcha stands for Completely Automated Public Turing test to tell Computers and Humans Apart. It is basically what it sounds like it is. It’s a test to see if the user is human or not. It prevents spam and automated data downloading. It uses simple tests that computers can’t solve like the distorted letters test or it may ask you to point out the street signs in a picture. Since both of the tests require the ability to analyse and comprehend contextual patterns,
There’s not much to explain for captchas. Let’s move on.
Here’s the wiki page for this.
reCaptcha is the same thing as Captcha except it has a double purpose of digitising books. There are several ongoing projects that are trying to convert books to digital text. They use a software program to scan the book and convert it to digital text. However, they can’t be quite sure that the program works correctly so they employ the help of humans. They show two words that can’t be read by computer programs. The first word is confirmed to say what it’s supposed to say. The second word is an unconfirmed word. If the person gets the first word right, the system assumes that they’ll get the second word right as well. When enough people have entered the same result for the second word, then that second word then becomes confirmed.