Your decision when buying a head unit all comes down to which features are important - and which ones are not.
Unless otherwise stated, the features mentioned here apply to both CD and DVD head units. I'm intentionally ignoring cassette head units because they are basically obsolete in modern car audio systems.
Most head units have a built-in amplifier, but don't make the mistake of thinking that most manufacturer's claims of "high power" are enough to drive a decent car audio system. Look for an RMS or continuous power spec, not a peak power spec.
RMS stands for root mean square and is a complicated mathematical formula for determining how much power an amplifier can deliver on a continuous basis. Peak power is usually a measure of the highest amount of power an amplifier can deliver in short bursts - and if it runs in that state for very long, the amplifier or speakers its powering can be damaged. Therefore, an RMS or continuous power spec is the one that you want to pay attention to.
Usually when you see a head unit advertised as high power with, it's a peak-power spec. The continuous power spec, if not given, will be about half of that. This may be sufficient to power, say, one or two speakers at modest volume, but it is not enough to produce quality sound or drive a subwoofer.
A preamp output is a pretty common feature on most head units because most car audio amplifiers can only accept a preamp or line-level signal. Preamp output refers to the signal before it's been amplified. Some head units also have multiple preamp outputs, which allows you to easily hook up more than one amplifier and retain front-rear fading capability when adding a subwoofer, which means you can adjust the level of sound between the front and rear speakers.
High Voltage Output
Not to be confused with a head unit's high-level or amplified output, the high-voltage output refers to the preamp output of a head unit that feeds an amplifier. A high voltage output allows the amplifier to run more efficiently. Also, the higher the voltage of the preamp output, the better it can reject noise. Most lower priced head units typically have a line-level voltage of between 500 and 1,000 mV, but higher end head units have preamp voltage in the 4 to 7 volt range.
This is typically in the form of a 3.5-inch miniplug jack and allows easily jacking an MP3 player or any other auxiliary audio source into the head unit.
iPod integration goes a step further than a simple aux-in jack by allowing control of Apple's ubiquitous music player directly from the head unit itself, and displaying info such as artist, album, song, and playlist on the head unit's display.
Many car audio head units now feature a USB input. When used for iPod integration with a compatible head unit, a USB port also allows for better sound quality and slightly faster access speed than any other means of hooking up an iPod to a head unit.
MP3/WMA capability means that a head unit can read discs (CDs or DVDs) with MP3 or WMA music files burned on them, or can read files via a USB drive or other means.
Satellite Radio Control
Many head units let you easily add XM or Sirius satellite radio so that you can tune in stations via the head unit and read station information on the display.
A credit card remote, so-called because it's roughly the same size as a credit card, lets you easily access features on a head unit without taking your eyes off of the road. Even better is a remote that mounts on the steering wheel.
Skip Protection or Memory Buffer
Even when a disc skips, skip protection keeps the music flowing by storing a few seconds of music in a memory buffer.
The dual zone feature is found on DVD head units and allows setting up two different entertainment zones in a vehicle. For example, a video signal from the DVD player can be sent to screens in the rear and an audio signal can be sent to wireless headphones, while front-seat passengers listen to a different source, such as FM or satellite radio.
Auxiliary Audio/Video Input
Also found on DVD head units, auxiliary audio/video input allows you to add an outside A/V source, such as portable DVD player or a videogame console, to the system.
Presets allow you to store a certain number of radio stations so that you can push a button to instantly access your favorites. Many head units also have a feature that automatically loads the strongest stations in an area into a preset bank.
Seek and Scan Tuning
Seek tuning finds the next strongest station on the dial when you hit a button, while scan finds the next strongest station and stops at it for a predetermined length of time before moving on to the next strongest station on the dial.
Local/distant tuning allows you to adjust the sensitivity of the seek or scan functions so that the tuner only looks for strong local radio stations or weaker distant ones.
Mute and Attenuation
Mute and Attenuation is a button on the head unit or the remote (or both) that lets you instantly mute the audio rather than turning the head unit off if you quickly need quiet. Attenuation is similar but turns the volume down to a certain level instead of muting the system entirely.
Loudness control boosts certain bass frequencies to help overcome noise in the vehicle as well as to compensate for the ear's inability to hear such frequencies at low-volume levels.
Bass, Treble, Balance, and Fader
Bass, treble, balance, and fader are basic controls found on all head units. The former are simple tone controls that allow you to cut or boost bass and treble frequencies. Some head units also include a midrange control. This latter allows you to adjust the level of sound from left to right (balance) and front to rear (fader).
An equalizer built into a head unit allows boosting or increasing the volume and cutting or decreasing the volume of certain frequencies. Some heads include graphic equalizers that let you cut or boost frequencies within a set range. Others include more sophisticated parametric equalizers that not only allow cutting and boosting certain frequencies, but also varying the frequency ranges that are affected.
Many CDs come with artist, album, and song info embedded on the disc, and compatible head units will display the information when the disc is played.
Repeat, Random or Shuffle Play
Repeat, as the name implies, allows you to repeat a disc or track, whereas shuffle lets you mix up the order of the tracks on a disc.
CD Changer Controls
CD-changer control used to be an essential feature, but in the age of the iPod, CD changers are about as desirable as car phones. But many head units still have this feature and it usually doesn't cost extra.
Multicolor and Multifunction Displays
You need to be able to quickly and clearly see information on a head unit while driving down the road, and manufacturers have done an amazing job of making bright and easy-to-read displays. But you'll have to decide if you really need a display with 700 different colors or one that shows animated dolphins jumping out of the water. Some even allow you to upload a digital picture that can serve as a screensaver or wallpaper on the head unit's display.
The clock, well, lets you know what time it is. (But I'm guessing you knew that already, right?)
From Car Audio for Dummies, copyright © 2008 by John Wiley & Sons, Inc., Hoboken, New Jersey. Used by arrangement with John Wiley & Sons, Inc.