It's the current, specifically going through one's heart (Horace is right), that kills - I recall something like 10 milliamperes can cause the heart to stop. The voltage needed to cause that current can vary a lot, depending on how it's applied to the body. Going between one arm and the other (such as touching the voltage with one hand and having the other on a water pipe), or between an arm and a leg (as in touching a source of voltage/current with a hand while standing in water in a wet basement or bathroom floor) are the conditions that send the current through the chest and heart.
A car's alternator only gives out 14 volts or so, which most people can't even feel (unless it's across your tongue). The danger is that it can give 30 amps (and a car battery can give hundreds of amps) which can generate hot sparks and heat a small wire to temperatures that will burn you almost instantly - A car's battery voltage is unlikely to hurt you, but if you short it out bad things can happen. Batteries can explode if they have too much charge or discharge current (or even "just because"). Stand back from both vehicles when you jump another car.
The wires from the spark coil to the distributor and spark plugs, however, can definitely shock you, and possibly electrocute you.
I recall that 48 volts is used as the "open circuit" (on hook) voltage for landline telephone systems (the technical term is "pots" - plain old telephone system - really!) because it's the maximum voltage that is (or was at the time) considered "safe" if you touch it. But when the phone rings there's a much higher voltage, about 100-120 volts, and at current that I'd guess is enough to kill.
If I had to do this, I'd say 100 volts across any two fingers of one hand would definitely cause lots of pain with very little chance of death, but the same voltage from one hand to the other may well cause death, but there's no guarantee it would kill. It may take several hundred volts to kill someone. Electric chairs used in executions put out a lot of voltage and power (surely there's info on the web on this), and I've heard of a case where one didn't immediately kill. Thus it apparently puts one through "too much punishment" and is "cruel and unusual," so they don't use electric chairs anymore.
How do I know these things? Mostly because I know about SAFETY around electricity. The general idea is to AVOID being killed.