I just ran into this article: Arduino: What adapter?.
It says that the external power supply should be 9-12V. That really surprises me, since my Arduino Uno can also be powered via USB, which is 5V.
Later he talks about a 9V pin. But I can only see a 5V and 3.3V pin on mine. Is he talking about a different version of Arduino?
Judging from the picture, his Arduino looks slightly different from my Arduino Uno R3, but I can't tell which version is on the picture.
The board has a voltage regulator which converts 9-12V to the 5V that the chip uses.
Why does it need 9V if the board works at 5V? Well, it doesn't need 9 exactly, but it needs something greater than 5. The reason for this is that the regulator has whats called a dropout voltage.
Looking at the schematic of the Uno we see it uses a NCP1117, which has a dropout of 1.2V. So the minimum voltage you can give it to get 5V is 6.2.
The 9V pin is the Vin pin which is at the same level as the input voltage (before converting).
And the third part of your question: It is possible to bypass the voltage regulator and feed REGULATED 5 volts directly to the 5V pin.
That's how the Arduino is powered using USB. The USB power is already regulated 5V, and is fed directly to the 5V pin.
Beware of doing this yourself however. You better be certain the 5V you're feeding in is a clean, filtered and regulated 5V. If it's not you run the risk of destroying your Arduino (and possibly anything connected to it's 5V lines.)
