Audio purists may blanch at the following, but if you want the convenience of instant iPod "integration" without the need for the modifications and expense of a factory-installed or sophisticated aftermarket unit, you can opt for one of the plug-in FM transmitter solutions which let you become your own personal "radio station", sending your tunes directly to your sound system via a neutral FM band.
The best of this genre usually combines an iPod caddy with an FM transmitter and plugs into your auxiliary power (cigarette lighter) socket, so it also charges your iPod while you drive (eg, the Griffin RoadTrip and the DLO Transpod).
While generally quite effective, totally transportable from car to car and relatively cheap (around $150-$180) the drawback of this system is that in heavily built-up areas it can sometimes be hard to find a clear "blank" spot on the FM band to tune into, and your sound quality may suffer.
Head of the class
iPods aside, the part of your sound system that you will deal with most is the head unit. This is your main interface to whatever you have hooked up to the system. It is from here that you switch between modes (radio, cassette, CD, minidisc, MP3 player etc), control volume, balance, fading and the general sound geometry of your system.
Head units these days come in an amazing variety of looks and configurations, often with dazzling animated front display units, some in full colour. Go to any car audio retailer or electronics store and you will usually find an extensive array of them to compare and choose from.
Some of them are real showstoppers, with an array of lights, buttons and display options that would stump a jet pilot. Others have a slightly more understated look. Most will have similar basic functionality but with head units you need to find the look that suits your car (and personality) as well as the features you need.
Be prepared to spend some time playing with the display units and comparing look, feel and functions with others. Most importantly, if you are mixing and matching the components of your whole system to get the exact sound capacity and features you need, you will have to make sure that the head unit and other components are all fully compatible and will work happily together once you have assembled the system in your car.