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 OS. The board has built-in Ethernet and WiFi support, a USB-A port, micro-SD card slot, 20 digital input/output pins (7 of them can be used as PWM outputs and 12 as analog inputs), a 16 MHz crystal oscillator, a micro USB connection, an ICSP header, and 3 reset buttons.
The Yún distinguishes itself from other Arduino boards by its ability to communicate with the Linux distribution onboard, offering a powerful networked computer with the ease of an Arduino. In addition to Linux commands like the cURL, you can write your own shell and python scripts for robust interactions. The Yún is similar to the Leonardo with the ATmega32u4, except that it has Linux on board. (has built-in USB communication, eliminating the need for a secondary processor).
The Linino OS installation occupies around 9 MB of the 16 MB available of the internal flash memory. You can use a micro SD card if you need more disk space for installing applications.
The Yún has a number of facilities for communicating with a computer, another Arduino, or other microcontrollers. The ATmega32U4 provides a dedicated UART TTL (5V) serial communication. The 32U4 also allows for serial (CDC) communication over USB and appears as a virtual com port to software on the computer. The chip also acts as a full speed USB 2.0 device, using standard USB COM drivers. The Arduino software includes a serial monitor which allows simple textual data to be sent to and from the Arduino board. The RX and TX LEDs on the board will flash when data is being transmitted via the USB connection to the computer.
The Yún appears as a generic keyboard and mouse, and can be programmed to control these input devices using the Keyboard and Mouse classes. The onboard Ethernet and WiFi interfaces are exposed directly to the AR9331 processor. To send and receive data through them, use the Bridge library. The Yún also has USB host capabilities through Linino OS. You can connect peripherals like USB flash devices for additional storage, keyboards, or webcams. You may need to download and install additional software for these devices to work.
Note: This version is without PoE.
Note: Yun Release 5. The new Arduino Yun R5 differs substantially from the previous release in these features; The power supply system provides 5V on AREF; The layout has been modified, adding two holes for USB signals and two holes for GP6 and GPIO13(LED2).