Arduino is an open-source physical computing platform based on a simple i/o board and a development environment that implements the Processing/Wiring language.
An Arduino can be used to develop stand-alone interactive objects or another option is to connect it to the software on your computer (e.g. Flash, Processing, MaxMSP). The open-source IDE can be downloaded for free (currently for Mac OS X, Windows, and Linux). Now moving on to the Arduino Yun, it’s like a member of the Arduino family.
Also, it’s the first member of a new groundbreaking line of WiFi products combining the power Linux with ease of use of Arduino. The first Arduino Yun is the combination of a classic Arduino Leonardo (based on the Atmega32U4 processor) with a WiFi system-ona-chip running Linino (a MIPS GNU/Linux based on OpenWRT). Arduino embedded the Linux machine directly on the PCB of the Arduino Leonardo and connected the two so that from Arduino it’s very easy to run commands on the Linux side and use it as an Ethernet and WiFi interface. Check out our Arduino Buying Guide! And note that in the meantime, the Arduino PoE will not work with this board, only a YUN will, so stay tuned.
Product pictures and description are of Sparkfun.com