Arduino Yun board
The Arduino Yun board has two processors on board the ATmega32U4 processor and the Atheros 9331. The latter runs a distribution of Linux called Linino, based on OpenWrt.
The board has built-in Ethernet and WiFi support. In addition to these main points, the Arduino Yun board also features , 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. As you realise, the Arduino Yun board is unlike any other boards. The following figure shows schematic for the Arduino Yun board.
Linux supported Wifi on the Arduino Yun board
The Arduino Yun board distinguishes itself from other Arduino boards in that it can communicate with the Linux distribution onboard, offering a powerful networked computer with the ease of Arduino. In addition to Linux commands like cURL, you can write your own shell and python scripts for robust interactions.
The Yun is similar to the Arduino Leonardo board in that the ATmega32u4 has built-in USB communication, eliminating the need for a secondary processor.
The Bridge library facilitates communication between the two processors, giving Arduino sketches the ability to run shell scripts, communicate with network interfaces, and receive information from the AR9331 processor.
The USB host, network interfaces and SD card are not connected to the 32U4, but the AR9331, and the Bridge library also enables the Arduino to interface with those peripherals.
The Schematic & Reference Design are available in a PDF format : Arduino-Yun-schematic.pdf