It may seem odd that I mention this first, but I want to get this out of the way. Messages Beta is the first application released to the general public from Apple’s Mac OS X 10.8 Mountain Lion.
Messages replaces iChat, but retains iChat’s services/information. More importantly, it unifies instant messaging/communications across Apple’s two platforms – Mac OS X and iOS. It’s the Mac OS X version of iMessage, similar to what iPad, iPhone, and iPod touch users running iOS 5 have.
Some of the features of Messages
* Unlimited iMessages sent between any Mac or iOS 5 device
* Conversations can be continued between Macs, iPads, iPhones, and iPod touches
* Photos, videos, documents, and other information can be sent.
* Can launch FaceTime video calls
* Supports iMessage, AIM, Yahoo!, Google Talk, and Jabber accounts/protocols
It is true that there are already cross-platform messaging systems for Macs (and Windows) such as Skype, but this is much more heavily integrated with the OS, and a little more intuitive across multiple platforms.
Link/Source: Mac OS X Mountain Lion – Messages Beta