Build quality:

Some head units are flimsy and weak. Some are built to last. Usually the best way to determine this is by the weight of the unit. Also, check to see if anything doesn't "fit", or if there are parts that are cheap or flimsy. The truth is this: The bigger name units tend to be better. Also, the more expensive units tend to be better. This isn't always a big deal for everyone, but I believe that build quality is very important.

Basic Head Unit Features:

Power output:

Most head units have built-in amplifiers. These drive the speakers. Some head units do not have internal amps, and therefore, require the use of an external amp. Head units without amps are usually high-end models. Anyway, power is measured in WATTS. We'll talk more about watts later, but for now, just see more watts as more volume. Most head units (unfortunately) are deceptive about the power output. Usually, the power output spec on a unit will say "50Wx4 MAX output". This means that when the laws of physics are on your side, it's a clear day everywhere in the world, the planets are aligned, the bass hits really really low and hard, and for about a tenth of a second, yeah, your head unit will put out 50Wx4. However, most of the time, your head unit is probably putting out a quarter of that. It's not that the head unit manufacturer is lying, it's just that they are exaggerating the truth, AND MOST PEOPLE DON'T KNOW BETTER. So, anyway, many people put a lot of importance on a head unit's power output. I don't because head unit amps suck, and no serious car audio person uses the head unit's amp anyway. Most head unit brands have about the same power output anyway. 9 times out of 10, an aftermarket head unit will put out more power than your factory head unit, and as such, will sound better.

RCA Preouts:

These are used to connect external amps to your head unit. These are the same red and white cables that you use for your home system. Basic head units usually have one set of preouts (front left, front right). Some head units have two sets (front left, front right, rear left, rear right), but most decent head units have 3 sets (front left, front right, rear left, rear right, subwoofer left, subwoofer right). One quick note about subwoofer outputs: Sometimes they're called "non-fading" outputs because they are not effected by the head unit's fade control (front to back balance). If you're not planning on adding an external amp, you don't need to worry about RCA pre-outs. If you want to add an external amp (or amps), I would always recommend getting a unit with three pre-outs.

RCA Pre-out voltage:

Usually this spec is next to pre-out spec. It is measured in VOLTS (which we'll go over later). Basically, the higher the voltage, the LOUDER the music will be at your amp. Don't look at this as "free power" because it is not. In addition, the higher the voltage, the less noise that will be introduced to your system. Again, this spec doesn't matter if you don't plan on using an external amp. If you do plan on using an external amp, try to find the HIGHEST voltage you can. 2 volts is basic, 4 is standard for a good unit, sometimes you'll see units with 5 volts. I know Eclipse units have 8V(!) pre-outs. In short, more voltage=less noise and cleaner sound.

Digital time correction:

This is used to simulate a perfect stereo environment. If you're at home listening to music, you want to situate yourself in the middle of the two speakers to get the best sound. The same is true in the car, EXCEPT that it's a hell of a lot more difficult. Usually, the passenger side speaker will be farther from your ear than the driver side. Since sound travels fairly slow (comparfed to light), you'll have one ear receiving a portion of sound before the other ear. The digital time correction delays the music slightly so your ears will be perceiving the same music at the same time. This is a very important feature if you are planning an SQ system.

Crossover:

This feature is incorporated into many components in a car audio system. If your head unit doesn't have it, don't worry. Just be sure your amps have it. Basically, this makes sure the right speakers are playing the right sounds. I will go into more detail about crossovers later.

Equalizer aka EQ:

This feature allows you to tweak the sound. The most basic EQ is a two-band, and controls BASS and TREBLE. Some head units have 3-bands, BASS, MIDS, TREBLE. More advanced head units have 5-band EQs. If you are serious about tweaking your sound to perfection, you won't use your head unit's EQ; You will get an external unit, and some of these units have 30 bands! I will go into much more detail about EQs later.

Theft Deterents/Security Features:

There are a few options when it comes to head unit security:

Detachable Faceplate:

The most common method is a detachable faceplate. Basically, you remove the faceplate of the unit (where the screen and controls are), and the unit won't function. This will deter thieves, but please don't leave the faceplate in your car. Thieves know most people do this, and will still break into your car, find the faceplate and steal your head unit