user578: I need to communicate between two Arduinos using XBee. I am new to XBee and hardware. The two Arduino Unos need to send a message (number) to each other and light an LED or play a sound, when they come close within a 10 meter distance.
So my doubt is which XBee shield and XBee module I need to purchase. I read about XBee series 2, XB24 etc.

user58: The "XBee" brand has two major types. Series 1 are the original ones, and are often NOT referred to by their series number. Series 2 are the newer mesh network brethren.
Most XBee's and their clones are pin compatible (there is even a Bluetooth replacement 'Bee), thus any XBee shield or adapter can be used for either series.
The difference is largely in their communication protocols. The Series 2 allow more robust mesh networking options. But for a beginner - the original (S1) provide simpler out-of-the-box networking.
There are retail packages available that include a pair of S1 XBees, an Arduino compatible breakout board, and a USB adapter to program them. This allows you to get "on the air" and setup the XBees quickly. For communication between two arduinos, a pair of shields will make physical connectivity much easier.

user11: The only way I can think of accomplishing this without some sort of proximity sensor is to measure the signal strength of the XBees.
From page 41 of the datasheet:

DB (Received Signal Strength) Command
AT Command: ATDB
Parameter Range [read-only]:  0x17-0x5C (XBee), 0x24-0x64 (XBee-PRO)
 DB parameter is used to read the received signal
  strength (in dBm) of the last RF packet received. Reported values are
  accurate between -40 dBm and the RF module's receiver sensitivity.
  Absolute values are reported. For example: 0x58 = -88 dBm (decimal).
  If no packets have been received (since last reset, power cycle or
  sleep event), “0” will be reported.

You can use this as a rough estimate of the distance between two sensors. You will have to correlate signal strength to actual distance and obstructions and other factors will cause the reported value to be off.
How to measure XBee signal strength?
This page has an example using Processing (on the computer): XBee Signal Strength

578: <p>I need to communicate between two Arduinos using XBee. I am new to XBee and hardware. The two Arduino Unos need to send a message (number) to each other and light an LED or play a sound, when they come close within a 10 meter distance.</p>

<p>So my doubt is which XBee shield and XBee module I need to purchase. I read about XBee series 2, XB24 etc.</p>
 58: <p>The "XBee" brand has two major types. Series 1 are the original ones, and are often NOT referred to by their series number. Series 2 are the newer mesh network brethren.</p>

<p>Most XBee's and their clones are pin compatible (there is even a Bluetooth replacement 'Bee), thus any XBee shield or adapter can be used for either series.</p>

<p>The difference is largely in their communication protocols. The Series 2 allow more robust mesh networking options. But for a beginner - the original (S1) provide simpler out-of-the-box networking.</p>

<p>There are retail packages available that include a pair of S1 XBees, an Arduino compatible breakout board, and a USB adapter to program them. This allows you to get "on the air" and setup the XBees quickly. For communication between two arduinos, a pair of shields will make physical connectivity much easier.</p>
 11: <p>The only way I can think of accomplishing this without some sort of proximity sensor is to measure the signal strength of the XBees.</p>

<p>From <a href="https://www.sparkfun.com/datasheets/Wireless/Zigbee/XBee-Datasheet.pdf" rel="nofollow">page 41 of the datasheet</a>:</p>

<blockquote>
  <p><strong>DB (Received Signal Strength) Command</strong></p>
  
  <p><strong>AT Command:</strong> ATDB</p>
  
  <p><strong>Parameter Range [read-only]:</strong>  0x17-0x5C (XBee), 0x24-0x64 (XBee-PRO)</p>
  
  <p> DB parameter is used to read the received signal
  strength (in dBm) of the last RF packet received. Reported values are
  accurate between -40 dBm and the RF module's receiver sensitivity.
  Absolute values are reported. For example: 0x58 = -88 dBm (decimal).
  If no packets have been received (since last reset, power cycle or
  sleep event), “0” will be reported.</p>
</blockquote>

<p>You can use this as a <strong>rough</strong> estimate of the distance between two sensors. You will have to correlate signal strength to actual distance and obstructions and other factors will cause the reported value to be off.</p>

<p><a href="http://forum.arduino.cc/index.php?topic=151145.0" rel="nofollow">How to measure XBee signal strength?</a></p>

<p>This page has an example using Processing (on the computer): <a href="http://www.makingthingstalk.com/chapter8/22/" rel="nofollow">XBee Signal Strength</a></p>
