I've read about the resistor drop thing- so we is it I can't use a resistor to drop the 12V from a car battery to 5V so that it can be used to power a GPS or other 5V electronic device?

If the power (current) draw is constant you can use a resistor, but if the current draw varies, so will the voltage dropped across the resistor - and thus the voltage across the item you're powering. For instance, a portable CD player will consume different amounts of current depending of whether the motor is spinning or not, or how loud you turn up the volume.

A string of diodes (connected in series) offers a slightly more useful solution. If the item in question is designed to operate from small drycell batteries (ie AAA, A, etc., it already accepts a fair range of input voltage (voltage of fresh vs almost totally depleted - but still runs the item - batteries). Each diode typically drops 0.7 to 1V, across a fair range of current, and the sum of voltages, and the variance, can be enough to power a device. An old portable CD player I bought ran on 4 AA cells, but also came with a car adapter with simply had 7 1N4001 diodes in series inside.

A 78xx regulator isn't more efficient than a resistor solution, it just monitors and regulates the output voltage which offers a stable output voltage - it'll still turn to heat the product of the dropped voltage and the current (P=U*I) the device uses, plus a little extra used by the regulation circuitry.

To get better efficiency you need a switching power supply (essentially chopping up the DC into AC, feed that to a transformer and then use the output AC from the transformer to create DC again, at another voltage level). A switching power supply can both step up or step down voltage - a resistor or diode solution only step down. A switching power supply can use either AC or DC as input (with AC input, just skip the initial chopping up the DC stage). You can then regulate the input side so that you get "just enough" voltage and current on the output side.

/Michael