In chapter four, they want you to use a pushbutton to control the LED. They tell you to build the circuit in figure 4-6. It shows using a 10k Ohm resistor on the negative side of the pushbutton. Either I am missing something, or forgetting something (most likely), but why do we need that resistor? Is it to protect the pushbutton, or to protect the input pin on the board?
the concepts demonstrated here are about the most important thing to learn about electronics when working with the Arduino:
1. An unconnected pin (also called floating input) is undefined and will give you random values when read. That's why the pin is connected either to +5V or Ground. Then you'll get HIGH or LOW properly.
2. A short circuit between +5V and Ground is bad. It will fry your power suply or regulator or other things you care about on your Arduino. Always make sure there's a component between +5 and Ground and there's no direct connection of those two. And that component in-between should limit the current to reasonable levels. The easiest way is to add a resistor.
And by the way, push-buttons and resistors don't have a positive or negative side, you can mount the either way. Diodes and LED have and need to mounted the right way or they won't do what you expect.