user144: There are some pins on the Arduino which I haven't been able to find out anything about:

IOREF
AREF
An unlabeled one next to IOREF

What are they?

user89: Concerning the AREF pin, you have to remember that the Arduino comes with a 10bit ADC (Analog-Digital-Converter), which converts incoming voltages between 0V and 5V to integer values between 0 and 1023. This results in a resolution of roughly 4.8 mV.
If a sensor only delivers a lower maximum voltage, it is resonable to apply this voltage to the AREF pin, just in order to obtain a higher resolution.

user8: AREF:
This is the voltage reference analog-to-digital converter (ADC). It can be used instead of the standard 5V reference for the top end of the analog spectrum – for example, if you wanted to use the ADC to monitor a signal that had a 0-1.5 volt range you could get the full scale of the ADC by connect AREF to a 1.5V signal. DO NOT CONNECT A SIGNAL OUTSIDE THE 0V TO 5V RANGE!
Note that in order for this to work, you must run analogReference(EXTERNAL); before using analogRead(). Also:

After changing the analog reference, the first few readings from analogRead() may not be accurate.

Fore more information, see AnalogReference.
IOREF:
This is a voltage corresponding to the i/o of that board, for example an Uno would supply 5v to this pin, but a Due would supply 3.3v. Sending a signal to this pin does nothing.
Unlabeled pin:
This pin is unused, but is there to ensure compatibility with future products. It is not connected to anything on any R3 boards I have seen.

144: <p>There are some pins on the Arduino which I haven't been able to find out anything about:</p>

<ul>
<li><code>IOREF</code></li>
<li><code>AREF</code></li>
<li>An unlabeled one next to <code>IOREF</code></li>
</ul>

<p>What are they?</p>
 89: <p>Concerning the <code>AREF</code> pin, you have to remember that the Arduino comes with a 10bit ADC (Analog-Digital-Converter), which converts incoming voltages between 0V and 5V to integer values between 0 and 1023. This results in a resolution of roughly 4.8 mV.</p>

<p>If a sensor only delivers a lower maximum voltage, it is resonable to apply this voltage to the <code>AREF</code> pin, just in order to obtain a higher resolution.</p>
 8: <p><strong>AREF:</strong></p>

<p>This is the voltage reference analog-to-digital converter (ADC). It can be used instead of the standard 5V reference for the top end of the analog spectrum – for example, if you wanted to use the ADC to monitor a signal that had a 0-1.5 volt range you could get the full scale of the ADC by connect AREF to a 1.5V signal. <strong>DO NOT CONNECT A SIGNAL OUTSIDE THE 0V TO 5V RANGE!</strong></p>

<p>Note that in order for this to work, you must run <code>analogReference(EXTERNAL);</code> before using <code>analogRead()</code>. Also:</p>

<blockquote>
  <p>After changing the analog reference, the first few readings from analogRead() may not be accurate.</p>
</blockquote>

<p>Fore more information, see <a href="http://www.arduino.cc/en/Reference/AnalogReference" rel="noreferrer">AnalogReference</a>.</p>

<p><strong>IOREF:</strong></p>

<p>This is a voltage corresponding to the i/o of that board, for example an Uno would supply 5v to this pin, but a Due would supply 3.3v. Sending a signal to this pin does nothing.</p>

<p><strong>Unlabeled pin:</strong></p>

<p>This pin is unused, but is there to ensure compatibility with future products. It is not connected to anything on any R3 boards I have seen.</p>
