For really old cars, the old audio cd format will do. It's a certain type of cd that reads like a vinyl and with only *.cda files to mark the start and end of each track. You can burn one using Nero and other similar tools.
Newer cars may support *.mp3, *.cda, *.midi, or even *.flac.
A simple/older auto CD player will only play Redbook CD's--Compact Disc Digital Audio (CDDA or CD-DA). These are CD's that have a sample rate of 44.1 KHz and a bit-depth of 16. If you are ripping disks for your car's audio system then lossless audio with Redbook specs will have *.aiff or *.wav extensions. iTunes and Windows Media Player can do this for you.
You can get audio DVD and SACD players for your car, but realize that you cannot rip SACD disks. That being said, would one really want to haul SACD's around in a car given their price? Some players will play lossy formats such as *.mp3, etc. and various lossless compressed files such as FLAC.
IMO, the best option is to get a nice CD player that has a USB interface and/or a SD card port. Put all of my lossless audio on an iPod or its equivalent, and then just plug it into the player. Using a SD/MMC card port gives one the opportunity to build an audio-mobile library using the cards.