iPhone 4 has two built-in cameras, one on the front above the display and one on the back next to the LED flash. The front camera has been tuned for FaceTime. It has just the right field of view and focal length to focus on your face at arm’s length. So it always presents you in the best possible light.
Imagine your sister is away at school and can’t make it to your son’s birthday. So you’re cheering her up with a FaceTime call. As the cupcakes come out, your son’s eyes light up, and you just have to share it. Tap a button, and before you can say, “Make a wish,” iPhone 4 switches to the back camera, and to the birthday boy's big moment. Another tap switches back to the front camera and to you. Simple, 
fast, and fun.