I know if you touch both terminals of a car battery 0 will happen to you but what would happen if someone started the car inadvertently while you were doing so? Wouldn't a large part of the 400 or so amps needed to turn the engine over go through you too?
The other answers failed to explain clearly why it won't shock you.
A car battery is a device that wants to keep voltage on its terminals at about the same level, about 13 volts. It will provide whatever current is required to do so. The internal resistance is extremely low, meaning it can provide several hundred amperes of current. A nearly ideal constant voltage source, so as to say.
50 milliamperes, less than thousandth of the current a car battery can provide may kill you. Why isn't car battery capable of putting that current through a human then? The answer is voltage and resistance. According to Ohm's law, current is voltage divided by resistance. Humans have a very high resistance, meaning voltages of over 50 volts are required to provide deadly current. A car battery has only 13 volts.
The starter motor, on the other hand, has much lower resistance. This means a car battery can put hundreds of amperes through the starter motor.
However, car battery has enough energy and power to kill. Just put a DC-DC converter with great enough output voltage to the battery, and the output will kill you. In fact, there may already be a somewhat similar component in the car's ignition system that converts the 13 volts of the battery to thousands of volts for the ignition system.
So, while some people say it's not the voltage that kills but the current, that is a bit misleading as high current through a human body requires high voltage. So I would instead say it's the voltage that kills, and the current is merely the result of a voltage.
The other answers noted correctly the problems of metal short circuits between the terminals, so I'm not going to repeat advice about the dangers of doing that.
The shock is not the problem
12 Volt is not considered harmful on intact skin. Dangerous DC starts at about 50V (assuming contact on dry, intact skin).
Short circuits are dangerous
Should you allow an short circuit to happen it will heat things up and produce sparks that could provoke fire. An often underestimated danger comes from body jewelery (best example: wedding ring) that could short the battery. A car battery has more than enough power to heat a wedding ring glowing red, resulting in a possible loss of your finger.
While starting, no, it wouldn't hurt you. In fact it would actually be slightly safer, as there would be a path between the - and + poles of the battery for electricity to flow that wasn't through you. Once the car was started, the voltage will increase from 12-13v to 13-14v, which still isn't enough to hurt you.
The real risk with automotive batteries is the explosive gas they produce and their acidic contents.
Electricity always follows the path of least resistance. All electricity wants to go to the ground. The car, being on rubber tires, is pretty well insulated from the ground. You, on the other hand, depending on what type of footwear you are wearing and what type of ground/floor you are standing on, are touching the ground.
Touching either terminal by themselves and absolutely nothing metallic about the car is safe. Touching the positive terminal and something metallic on the car, you are providing a path for the electricity to follow. The most common occurrence of this is wrenching near the battery and having your spanner or ratchet touch the positive terminal. Whatever you are wrenching on is metallic, so there you do, big spark, loud pop, your wrench or ratchet goes flying and gets really hot.