If you're considering upgrading your automobile's sound system, chances are your first step will be to replace the factory head unit with an aftermarket product. Provided you already have working speakers, this move can instantly improve the sound. There are several names for this component of the system, and one is "head unit," which applies to any dash-mounted interface that controls car audio.
Varieties of Head Units.
Tuner - A unit that lacks built-in power amplification. Tuners require an additional amplifier to power the speakers.
Receiver - One that contains an onboard amp. A receiver can be used to power an existing set of speakers without other equipment.
Controller - An interface that has no radio function, although it may include any combination of CD, DVD, cassette, auxiliary and/or USB inputs.
Mechless - A receiver or tuner without CD, DVD, or cassette capabilities. "Mechless" refers to the absence of mechanical (moving) components, as these devices instead feature solid state construction. These pieces of gear can have auxiliary, USB, and Bluetooth® compatibility as well.
Some of these categories overlap. For example, controllers and mechless heads can exist with or without built-in amps. You'll have to read the specs of any equipment that interests you to make sure it meets your requirements.
What Do You Need?
Now that you know the basic distinctions, you've got to consider the factors that may influence your decision on what car audio equipment to install. If you just want to replace what's in the dash first, you'll need something with its own power to get sound out of your factory speakers. However, if you plan on upgrading your speakers and adding subwoofers and tweeters for an all-out overhaul of your car audio system, you will likely need an external amplifier to supply sufficient juice. In this situation, you may want to buy a receiver with preamp outputs. These outs will bypass the onboard power, causing less distortion so you can bump louder music without sacrificing sound quality. If sound quality is your focus and loudness is not important, you can save money by foregoing the amp and subs, using a unit with sufficient power and buying better quality door speakers.
Now think about getting into your car and being in the mood to blast your favorite songs. What would you reach for? Do you have a vast collection of CDs you'd like to slip in and listen to all the way through? If so, you definitely need a model with a disc player. Maybe you've ditched the hard copies, and your music is stored digitally on your smartphone or dedicated audio player. In this case, you may elect to go fully mechless and play your library through a USB connection for the best audio. If streaming services on your phone are your preferred method of music retrieval, an interface with dedicated Bluetooth is handy, or, at the very least, an auxiliary jack should be present.
With what you now know about the brain of your car audio system, you can prioritize the features most important to you and search for a product that offers them all within your budget.