user13972: I'm just wondering why the VIN would require 7-12 volts when the USB port itself only requires 5 volts? What happens if you power an Uno on VIN with 5 volts?

user313: USB's Vcc pin is assumed to be a stable, 5v supply, and is directly connected to the 5v bus on the Arduino board with no further regulation. Vin is assumed to be an unknown and unregulated voltage in the range of 7-12 volts. 
The 7v minimum is a requirement of the design of the on-board voltage regulator which needs the 2v "head room" to maintain its output at 5v. A 5v power supply could be designed with a larger input range, even including voltages less than 5v, but the simple design used on the board covers a range of sources at a lower cost than a higher-performance power supply.

13972: <p>I'm just wondering why the VIN would require 7-12 volts when the USB port itself only requires 5 volts? What happens if you power an Uno on VIN with 5 volts?</p>
 313: <p>USB's Vcc pin is assumed to be a stable, 5v supply, and is directly connected to the 5v bus on the Arduino board with no further regulation. Vin is assumed to be an unknown and unregulated voltage in the range of 7-12 volts. </p>

<p>The 7v minimum is a requirement of the design of the on-board voltage regulator which needs the 2v "head room" to maintain its output at 5v. A 5v power supply could be designed with a larger input range, even including voltages less than 5v, but the simple design used on the board covers a range of sources at a lower cost than a higher-performance power supply.</p>
