IMessage is Apple's messaging service that lets iPhone, iPad and iPod Touch users message each other for free over a data connection (WiFi or cellular data). With the announcement of OS X Mountain Lion, Apple has brought iMessage to the Mac. A beta release of Messages app from Mountain Lion is now available for download from Apple's website. The download will run on Macs running OS X Lion only. The program cannot be installed on Mac OS X Snow Leopard or older. Messages is essentially iChat and iMessage for Mac combined in a brand-new interface.
All the iChat functionality - AIM, Google Talk, Yahoo! And Bonjour chat - is still there, though the iChat application itself is replaced by Messages. You can download the Messages beta from. Usual caution associated with beta software is advised.