It breaks up into three simple parts:

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![schematic](http://i.stack.imgur.com/hZt69.png)

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The first part is the diode that provides reverse voltage protection. If for some reason the polarity of the input voltage is wired opposite to what it is supposed to be, then \$D_1\$ will block it and the output will also be essentially off. Only if the polarity is correct, with the rest of the circuit be operational. The price of including this is a voltage drop of perhaps \$700\:\text{mV}\$. I exaggerated this a little in the diagram. But it gets the point across.

The next section is below that. It's a zener regulator. The resistor is there to limit the current. The zener tends to have the same voltage across it, when reverse-biased with sufficient voltage (and \$12-14\:\text{V}\$ is more than sufficient.) With \$R_1\$ as given, you'd expect the current to be somewhere from about \$5\:\text{mA}\$ to \$10\:\text{mA}\$. This is a "normal" operating current for many zeners. (You could go look up the datasheet and find out, exactly. I didn't bother here.) So the voltage at the top of the zener should be close to \$9.1\:\text{V}\$. The exact current through the zener will have a slight impact on this. But not much.

The final section is there to "boost up" the current compliance. Since the zener only has a few milliamps to work with, if you didn't include this added section your load could only be a very small few milliamps, at most. To get more than that, you need a current boosting section. This is composed of what is often called an "emitter follower" BJT. This BJT's emitter will "follow" the voltage at the base. Since the base is at \$9.1\:\text{V}\$, and since the base-emitter voltage drop will be about \$600-700\:\text{mV}\$, you can expect the emitter to "follow" but with a slightly lower voltage (as indicated in the schematic.) The BJT doesn't require much base current in order to allow a lot of collector current to be added. So the BJT here "draws" current from its collector, adds that current to the tiny base current it uses, and then this becomes the total emitter current. This emitter current can be several hundred times the base current. So here, the BJT might draw \$1\:\text{mA}\$ of base current (which is okay, because there is several times that much available due to \$R_1\$) in order to handle perhaps as much as \$200\:\text{mA}\$ of emitter current. In keeping with the idea of "being conservative" the specification only says \$100\:\text{mA}\$ -- and that's very much the right way to go when telling someone what this is capable of. Be conservative.

\$R_2\$ is there as a bit of a short-circuit current limit. It doesn't serve much else. But if you try and pull too much current from the emitter on this device then there will be a large voltage drop across \$R_2\$ and this will cause the collector to have access to a much lower remaining voltage. So the emitter will be "cramped." Overall, \$R_2\$ is a very cheap way to add some modest protection to help make the whole thing just a little more bullet-proof, so to speak.