The Head Unit
A high-end home audio system uses separate electronic components — a radio tuner, CD player, preamp, amplifier — connected by various cables. Since space is at a premium in a vehicle, automakers have to cram as many of these components as possible into one device: the head unit. Because of this, the head unit performs multiple duties, but its two main functions are controlling the overall system volume and the various audio sources in a vehicle.
In addition to controlling the system's volume, head units usually include basic tone controls such as bass and treble to tailor the sound to the listener's taste. Many audio systems also include signal processing that automatically adjusts the volume, depending on the ambient noise in a moving vehicle. Some high-end OEM audio systems also have a separate subwoofer and subwoofer-level controls.
Auto audio systems are just more complex than they use to be. You used to hook up speakers to your own tape playing stereo, but nowadays there is a set of distinct speakers to set up with your car CD mp3 player, known as component speakers.