What is a CAPTCHA

What is a CAPTCHA?

A CAPTCHA is an automated test that can distinguish between machines and humans online. It can generate and grade tests automatically that accepts virtually all humans and rejects virtually all machines.

For example, human can generally read degraded images, but OCR machines cannot.

Why is it useful?

CAPTCHAs are designed to prevent bots - programs that pose as humans on the Internet- from abusing internet services. Bots, driven not to dominate but to sell, sign up for thousands offree email accounts every minute, sending millions of spam messages from them.They infiltrate chat rooms, collecting personal information and posting links topromotional sites. They generate worms, break password systems, invade privacy, and drain resources.

To defend e-commerce systems from bots, an increasing number of companies arearming themselves with CAPTCHAs. For example, users registering on Yahoo mustfirst correctly recognize a distorted word displayed against a clutteredbackground and type it into a box to prove they are human. Such reading-basedCAPTCHAs exploit the large gap between humans and machines in their ability toread images of text.