Your source unit or head unit it the main brain of the system from where you control your system from. There are a number of different systems on the market from FM, CD,DVD, USB, iPod, Bluetooth, Digital Radio, Meckless (NON-CD), single din and double din.
There are also interfaces allowing you to still use your steering wheel remotes and in some cases the information on the head-up display.
Even if you have a complicated dash, there are adapter plates and fascia panels that will convert your dash to either single din or double din.
Everything from FM RDS radio, AUX-IN on the front or rear of the unit, SD card storage for music, DAB Digital Radio, HDMI, DVD Video, USB devices, Bluetooth handsfree and music capability for both Android and Apples iPhone, Ipod and iPad products.