Essentially, its a next-generation Blackberry Messaging (BBM) service for the iPhone, iPad and iPod Touch.
iMessages has been designed as a cross-platform iOS app that will enable users of any iOS device to message each other, for free, on any network using 3G or Wi-Fi.
iMessages will support sending photos, videos in addition to text, in real-time between users. You can see, also in real-time, when someone is replying, as well as receive delivery receipts and optional read-receipts so you know when someone has actually read your message.
People will be quick to say that this will only be useful for those people whose friends have an iPhone, but that was never a limiting factor in the popularity of BBM, and I doubt it will limit the popularity of iMessages.
With over 200 million iOS devices out there, and SMS an exorbitantly over-priced dinosaur – iMessages is certainly going to change the way many iOS users communicate.