FaceTime allows you set a caller ID. If you only have one iPhone phone number, or iPod touch, iPad, or Mac email address associated with FaceTime, that'll be your caller ID. If you have several different numbers and addresses associated with FaceTime, however, you can choose one of them to serve as your consistent, unified called ID. That way, no matter which number or address you're FaceTime calling from, the person receiving the call will always see the same contact, and easily know it's you. If you have multiple iOS and/or OS X devices, you can either set them all to the same caller ID, or to different ones -- for example, business and personal -- so you can receive calls from any number or address, but always send them from the same, specific number or address.
That's all there is to it. FaceTime will now use that number or email as the consistent caller ID for that device.
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.