iMessage is Apple's new messaging service that is aimed to compete with RIM's Blackberry Messenger - sidestep text charges from cell phone providers and just go direct to the device.
In Apple's case, it's even better than BBM because the device can also be your Mac computer via iChat. So if someone sends you an iMessage, it'll pop up on whatever device you're currently logged in to. On BBM, if your Blackberry can't connect you don't get your messages.
Which brings up an interesting thought in how Apple is further side-stepping the cell phone providers, which John Gruber pointed out:
Very cool. And keep in mind that while your phone number changes with a pre-paid SIM, your iCloud ID doesn’t, so you can use iMessage to receive message from other iOS users.
I can cancel/stop using an iPhone, or change carriers or even countries, and still keep up with people who I used to iMessage with. I don't think the same option would exist for Blackberry users. As far as I'm aware, you have to have a Blackberry to communicate over BBM.