The Yún has two processors on board. One is an ATmega32U4 like on the Leonardo. The other is an Atheros 9331, running Linux and the OpenWRT wireless stack, which enables the board to connect to WiFi and Ethernet networks. It is possible to call programs or custom scripts on the Linux system through the Arduino to connect with various internet services. The Yún Shield shares the same architecture and features, but it is a shield and needs to be attached to a board, where the microcontroller is interfaced with the Atheros processor through hardware Serial port.
The Bridge library simplifies communication between the ATmega32U4 - or the board attached if you use the shield - and the AR9331. Bridge commands from the board microcontroller are interpreted by Python on the AR9331. Its role is to execute programs on the GNU/Linux side when asked by Arduino, provide a shared storage space for sharing data like sensor readings between the Arduino and the Internet, and receiving commands from the Internet and passing them directly to the Arduino.
Bridge allows communication in both directions, acting as an interface to the the Linux command line. For a brief explanations of the terminal and executing commands on Linux see here.
Process is used to launch processes on the Linux processor, and other things like shell scripts.
Console can be used to communicate with the network monitor in the Arduino IDE, through a shell. Functionally, it is very similar to Serial.
An interface to the Linux file system. Can be used to read/write files on the SD card, or on the USB memory if you are using the Yún Shield.
Creates a HTTP client on Linux. Acts as a wrapper for common CURL commands, by extending Process.
An asynchronous, session-less interface for communicating between Linux and Arduino.
An Arduino based HTTP client, modeled after the EthernetClient class.
An Arduino based HTTP server, modeled after the EthernetServer class.
An interface to Temboo making it easy to connect to a large variety of online tools. See the Tembo documentation for more.
Corrections, suggestions, and new documentation should be posted to the Forum.
The text of the Arduino reference is licensed under a Creative Commons Attribution-ShareAlike 3.0 License. Code samples in the reference are released into the public domain.