| More than a microprocessor, a System on Chip (SoC) is an integrated circuit where all of the subsystems involved are on a single piece of silicon. For example, an FM radio requires a tuner, an amplifier, and a power management circuit. Prior to the development of SoCs, one would need an microchip for each portion of the entire circuit, and they would be soldered on a printed circuit board that would make up the core of the device.
Advances in miniaturization and chip design have enabled the entire circuit to be placed on a single microchip. This integration has occurred in many application areas, from the aforementioned radio-on-chip to such complex devices as a camera-on-chip, which integrates on the same piece of silicon the image sensor and all related signal processing circuitry. That's why you can buy a cell phone with a camera in it today.
There is even research going on now (early 2003) using continuous-grain silicon that will enable the creation of System LCDs (SLCDs) that integrate all driver and operation circuitry--including digital logic, LCD driver, power supply, I/O interfaces, and signal-processing circuitry--onto the glass itself. Imagine a credit-card-sized PDA where the entire surface is a touchscreen display. Cool. | 
