It runs a lightweight Linux distro, and also combines WiFi and Ethernet connections on board for communicating with networks out of the box. Why? The Arduino people write:
Yún means “cloud” in chinese language, as the purpose of this board to make it simple to connect to complex web services directly from Arduino.
Basically, it has the same footprint as an Arduino Uno but there's an ATmega32U4 microcontroller (the same as the Leonardo) and also a Linux system based on the Atheros AR9331 chipset.
To program the Yún, you'll need to download the Arduino software, and select "Arduino Yún from the "Tools > Board" menu (according to the microcontroller on your board). Tutorials can be found online.
Because the Yún has two processors, the summary below shows the characteristics of each one separately.
|AVR Arduino microcontroller|
|Digital I/O Pins||20|
|Analog Input Channels||12|
|DC Current per I/O Pin||40 mA|
|DC Current for 3.3V Pin||50 mA|
|Flash Memory||32 KB (of which 4 KB used by bootloader)|
|Clock Speed||16 MHz|
|Ethernet||IEEE 802.3 10/100Mbit/s|
|USB Type-A||2.0 Host/Device|
|Card Reader||Micro-SD only|
|RAM||64 MB DDR2|
|Flash Memory||16 MB|
|PoE compatible 802.3af card support|
The Yún is actually meant to be the first member of a series of Wi-Fi products combining Arduino with Linux. We await its successors.