CarPC is a computer that runs in your car.  The idea is very basic, put a pc in your car.  But actually getting a working, SAFE solution is the hardest part.  Also, before you start building a CarPC system, first sit down and think about some of these questions:  

What will I use it for?  How do I want to be able to control it, via touch screen or keypads, or voice?  Will it run off the car's battery/electrical system?  GPS? Will I build the machine from ground up? Integration into your current car's interior, modding?

This will get you started in the right direction; the biggest challenges and concerns for a CarPC (to me at least) are: input, safety, power.