The Arduino Yún is a microcontroller board based on the ATmega32u4 and the Atheros AR9331. Use this tag for questions specifically regarding the Yún, and not just general Arduino usage.
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 Wi-Fi 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.
The main differences between the Yún and other Arduinos is that the Yún contains an onboard Wi-Fi 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.