Start reading: http://www.mp3car.com/vbulletin/advfaq.php
If the computer is in hibernate mode, it is drawing no power whatsoever.
If the computer is in standby mode, it is drawing enough power to maintain the system state in RAM. If the power is lost while in standby, the computer will do a full boot when the power switch is pressed.
For a vehicle install, desktop hardware is easier to set up, in my opinion. Most desktop hardware has the option for both standby and hibernate. Hibernate is the most commonly used option, as you get a faster startup time than a full boot, but are still drawing no power when shut off.
Laptops have other issues when configuring startup, namely getting the laptop to start automatically when the ignition goes hot.