iMessage lets you send absolutely free text (SMS) messages to anyone else using an iPhone, iPad, or Mac. It sounds simple, and it is, but there are a few important things to pay attention to.
Right now Apple uses two different systems for linking you to your iMessages. If you have an iPod touch, iPad, or Mac, your Apple ID email address will how you send and receive iMessages. If you have more than one Apple ID, iMessage will use whichever one you log into in Settings. Anyone you send an iMessage to will see it coming from that Apple ID, and anyone who wants to send you an iMessage will need to send it to that Apple ID.
If you have an iPhone, your iPhone telephone number will be used to identify use for sending and receiving iMessages, just like SMS and MMS messages on a regular phone. However, your Apple ID email address can be used as well, but will currently be treated separately.
What that means is, if someone iMessages your telephone number, it will only show up on your iPhone. If someone iMessages your Apple ID, it will show up on any iPhone, iPod touch, iPad, or Mac logged into that Apple ID.
If that sounds confusing, don't worry. With iOS 6, set for release this September, Apple will allow you to "merge" your iPhone phone number and Apple ID so all your iMessages will go to all your devices, regardless of whether or not they're sent to your phone number or email address.
If you're using an iPod touch, iPad, or Mac, you can only ever send iMessages. If you're using an iPhone, however, you can send both iMessages and standard SMS/MMS messages as well. (You need that in order to contact people who don't use Apple phones, including people on regular feature phones.)
To make it easier to tell the difference, Apple color codes all iMessages with blue bubbles, and all SMS/MMS messages with green bubbles. If you send a message and the bubble around it is blue, you're using iMessage. If the bubble is green, you're using SMS/MMS. If iMessage is offline, your iPhone may try to send over SMS/MMS, so keep an eye on it if you want to avoid charges, especially international charges outside of any texting plan you might have.
Once you're set up, you're ready to go. If you've ever sent a text message with your iPhone, you already know how to send an iMessage -- it uses the same Messages app. If you're brand new to the iPhone, or to iPod or iPad, here's how it's done.
That's all there is to it. Green bubbles represent actual text messages while blue bubbles represent iMessages. There's nothing for you to select or do, the Messages app handles it entirely on its own.
If you're having issues getting iMessage to activate, get more help at the iMore forums, and for more tips, check out: