You have 3 choices that I know of with 3 different sound qualities.
1. Ipod adapter that integrates with your HU. Usually plugs into the HU Changer port. This allows the HU full control of the Ipod and the Ipod Folder and other information to be displayed on your HU. Your HU must support this and most factory systems do not. Although some newer cars do have built in Ipod adapters.
2. If you car system supports an Aux input which many do you can plug a cable from your Ipod headphone jack to your Hu and select the Aux. This way makes you choose the songs using your ipod and not the HU. Good quality but not usually as good as option #1.
3. You can buy an Ipod adapter that plugs into your Ipod and sends the Ipod music out as an FM frequency signal. You would then tune your HU to the correct FM channel to listen to your Ipod. Not very good quality IMO.