Hershball,
as you see there are many models out there. Use Crutchfield, woofersetc and mainstreetaudio to get and idea of the different models and prices.
Main brands are Alpine, Kenwood, Eclipse, Pioneer. There are units that are double din like Mike's and there are units that are single din units where the screen slides out and then flips up like the Alpine D100 I have.
Most of these units play DVDs (only American region 2 though, darn it) and even DVD-Audio but not SACD. Of course, they all play CD and MP3. They can decode the Dolby digital and dts info on the DVD and downsample it to 2ch or 2.1ch. If you want 5.1 surround sound you need an extra processor.
Many have additional video outputs that you can use to show video on headrest screens for the back passengers. Many also have video inputs that can be used for navi or reaerview cameras.
The functions of the unit are usually accessed through the touchscreen and some extra hard buttons. Some units come with inputs for steering wheel control modules others need to use the universal PAC SWI-X module that works like a programmable remote control.
This type of DVD indash solution starts at around $450 or 500 and goes up to $2700 with built-in hard disk and navi.
If you have further questions, just ask.
Till