Messages is iMessage for Mac - Apple's replacement for iChat in OS X 10.8.
Firstly, note that if you install Messages, it replaces iChat although Apple has assured that iChat services will continue to work. In a further move towards iOS, Messages provides better integration with Messages on iPad, iPhone, and iPod touch running iOS 5.
Some of the benefits include unlimited Messages to these devices, the ability to start a conversation on your Mac and then continue it on your iOS device and launch FaceTime video calls. You can also send photos, videos, attachments, contacts, locations, and more.
If you want iMessage for Mac then Messages brings OS X and iOS one step closer to make sure you can always keep in contact with your friends and family.