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 accept virtually all humans and reject virtually all machines.

For example, humans 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 of free email accounts every minute, sending millions of spam messages from them. They infiltrate chat rooms, collecting personal information and posting links to promotional sites. They generate worms, break password systems, invade privacy, and drain resources.

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