What is use of CAPTCHA

What ?
It is a random generated feature using which you can avoid automated filling , programs etc.

Why?
It is used to verify User Authentication ( in the sense that the user is doing the thing instead of a program)

For more information see,

Captcha

Visual Studio 2010

Enhanced User Experience
SharePoint Development in Visual Studio 2010
Democratizing Application Lifecycle Management
Inspiring Developer Delight
Cloud Development
Web Development

For more information see,

Visual Studio 2010

Ecommerce Payment Gateways

www.paypal.com
www.e-gold.com
http://www.cybersource.com