While FaceTime was originally locked to an iPhone phone number, now that iPod touch, iPad, and Mac can make FaceTime video calls, Apple has added email addresses as a way to connect. Usually your FaceTime address will be your main Apple ID -- the one you use for iCloud, and sign in with when you setup your device for the first time. However, you can all register additional addresses with Apple and FaceTime. This makes it relatively simple to ensure anyone with any of your addresses can get through, or even to separate work from family addresses, or to use different addresses on different devices.
That's all there is to it. You'll now start receiving FaceTime calls to that email address as well!
If you don't want your iPad or iPod touch to receive FaceTime calls sent to your iPhone phone number, or don't want your work iPhone or Mac to receive FaceTime calls sent to your personal email address, you can easily disable -- or enable -- them on a per-device, and per-address/number basis.
To enable or re-enable numbers or addresses, simply reverse the process and tap the empty space to the left to number or address to add a checkmark.
If you need more help with using FaceTime on your iPhone, iPod touch, iPad, or Mac, check out the articles below, or head on over to the iMore forums.