On the bottom edge of the iPhone, Apple has parked three important components, none of which you'll ever have to bother with: the speakerphone speaker, the microphone, and, directly below the Home button, the 30-pin connector that charges and syncs the iPhone with your computer.
There's only one payoff for knowing what's down here: The speakerphone isn't very loud, because it's aimed straight out of the iPhone's edge, away from you. If you cup your hand around the bottom edge, you can redirect the sound toward your face, for an immediate boost in volume and quality.
On the back of the iPhone, the camera lens appears in the upper-left corner. On the original iPhone, the rest of the back is mostly textured aluminum; on the iPhone 3G, it's shiny hard plastic, in black or white. Cellphone signals have a hard time going through metal, which is why Apple switched to plastic. (All told, there are ten different radio transceivers inside the iPhone: four each for the standard GSM frequencies; three for the three 3G frequencies; and one each for Wi-Fi, Bluetooth, and GPS.)