FaceTime and iMessage are both really cool, and really confusing. Here's a quick rundown as to how to put them to work for you.
FaceTime is Apple's video call technology included with recent versions of OS X for Macs, and iOS for mobile devices. You see the person you're talking to, and they see you. Sort of like Skype. You can also use it for super high quality audio-only calls. Simple, right?
iMessage is Apple's text messaging system, which is also built in to recent versions of OS X and iOS. It's like cell phone text messaging, but you can have long messages, you can include attachments, it works over Wi-Fi, and you can verify that your message been delivered (and, optionally, you can communicate to the sender that you've received and/or read a message). Sort of like BlackBerry Messenger, for those of you who remember that. Also simple, right?
Where it gets confusing is that FaceTime and iMessage are available on your Mac, iPhone, and iPad, and you can be reached either at your email address, or your cell phone number -- on any of them. It's because your cell phone number and email address (or addresses) are all just different labels for getting an iMessage to you.
Let me go over that again: if you have Mountain Lion (10.8) or later on your Mac, and you have Messages open, and someone sends you a text message from their iPhone to your phone number...you'll see it pop up on your Mac. And you can respond on your Mac, too, and it will look like it's coming from your phone number. If you're signed in to iCloud on all your devices, they all communicate with each other; that's how your Mac knows what your phone number is.
And just as unfamiliar is the idea that if you send an iMessage from your phone, you can just as easily send it to someone's email address as you can their phone number. It will show up in the Messages app on their iPhone, iPad or Mac, not their email. (I used this recently to send a message to my cousin, whose mobile number I didn't have.)
This all applies to FaceTime as well. You can be "called" at your phone number and any email addresses you might have. And then you can also add any other arbitrary email address, if you wish, from within the settings. (Keep in mind that none of this has anything to do with your actual phone or email service. Again, your phone and email addresses are just labels for reaching you via iMessage and FaceTime.)
What's also confusing is a lack of consistency. On a Mac or iPad, there's a FaceTime app; but there isn't one for an iPhone. Why? Only Apple knows. Instead, you just look up someone in your contacts, and click the FaceTime icon rather than his or her phone number. And in the Messages app, if the Send button is blue, then you are using iMessage with someone else who can receive it. If it's green, you're sending a plain old 160-character SMS text message.
If you don't know whether or not someone has FaceTime before you call them, how can you find out? Well, here's the trick: if you start writing them a text message, and you see that the send button is colored blue, they've got iMessage, and so they might have FaceTime as well. If the send button is green, it's a plain old SMS text message, and they probably aren't FaceTime-capable.
If your head is spinning, just try it out. Look up a friend who has an iPhone, and call him or her with FaceTime, rather than his or her phone number. It's fun. Just remember you're on camera.