iMessage was Apple’s newfound instant messenger service, which replaced iChat. The service was made available to users who run OS X 10.8 Mountain Lion or above. This service helped Apple to create a more seamless and highly integrated experience for users of iOS. When Apple released a developer preview of OS X 10.8 "Mountain Lion", it Also released Messages, which is just iMessage but for OS X.
If you install iMessage on your system, then the iChat service is replaced. However, this doesn’t mean that the iChat service will stop working, as assured by Apple. Moreover, the iMessage service integrates with the iPod Touch, iPhone and iPad that are running the iOS 5 or higher. This way, you get a better experience with all Apple devices.
It is possible to send unlimited iMessages to all the iOS devices. You can even start a conversation on the iPad and continue it over your Mac or vice versa as everything is largely integrated and that includes FaceTime. Moreover, the usual features expected in this kind of service like sending contacts, location, videos, pictures and other attachments are allowed here as well. iMessage also allows you to connect to other chat services such as Yahoo!, Messenger, Google Talk (now Hangouts), Jabber and AIM. iMessage is the perfect service if you own multiple Apple devices that are running OS X and iOS.
Interface wise, it is classy and clean with a perfect design. As the flagship Apple messaging service, it is the perfect instant messenger that will ensure you stay in touch with family and friends.