If you really like to see digital TV in your car whlie driving (I assume, for law, you are on back seat ), then you should look for tuner, that has word "diversity" somewhere in description. This means, that the tuner has actually 2 tuners inside and it has 2 antenna connections like on the above picture from user "blk02si". You should connect both antennas and point them to different directions.
This is needed for good reception, tuner measures signals from both antennas and decides which is best to show you.
I've seen USB diversity tuners, that have one internal antenna and second you can put on roof or wherever. Also some computer connected tuners have capability to show one tv-programme and record another on same time, but with tihis function you loose diversity function.