In deference to mobile phones, the faceplate is the plastics that cover the "front" (business side) of the handset. They usually cover the speaker, microphone, display and keypad with appropriate cutouts for each.
The term has only recently become a going concern to the general public with many cellular and mobile phone companies beginning to offer phones with user removeable (and thus, interchangeable) faceplates. Though this could be reasoned as way to replace a scratched or broken faceplate, the main reason people will replace a faceplate is to personalize their phone to their, uhm, personality.
The first company to do this on a comsumer scale of any recognition was Nokia in the Mid-1990's, though interchangeable faceplates have been around (albeit not user changeable) for OEM and distributor equipment for some for before that for branding reasons (though these are not counted really since it required disassembling the entire phone).
Most modern phones will have a faceplate that can be removed and reapplied with minimal ease with the use of tabs and clips; screws are seldom used anymore. In addition the the faceplate being replaced, you my find that the button layout on the new faceplate is differnet than the existing layout and may require a replacement keypad as well (the Nokia 5160/5180i phones are notorious for this, as their keypad style is not used by any other phone on the 51xx line).