The Arduino Yún is a microcontroller board based on the ATmega32u4 and the Atheros AR9331. The Atheros processor supports a Linux distribution based on OpenWRT named Linino. The board has built-in Ethernet and WiFi support, a USB-A port, micro-SD card slot, 20 digital input/output pins (of which 7 can be used as PWM outputs and 12 as analog inputs), a 16 MHz crystal oscillator, a micro USB connection, an ICSP header, and a 3 reset buttons. (From http://arduino.cc/en/Main/ArduinoBoardYun)
The main differences between the Yún and other Arduinos is that the Yún contains an onboard WiFi antenna used to program the board wirelessly, connect to the internet in sketches, or see various information about the board. The Yún also contains a Linux side, which is different because most boards only have one onboard processor. The Yún has two onboard processors; one for the system and one for the Linux part. One can connect to the Linux side of the Yún via a Bridge library. Python 2.7 is also included with the Yún.
Only use this tag if your question is specific to the Yún, and not a general Arduino question. Also consider using other related tags such as programming or pins more specific to your problem.