It is quite a little improvement over the basic Arduino UNO. I have only begun to explore the possibilities, but it looks like it will be fun.
Yun is based on the ATMega32u4 microcontroller, but also is distinguished by the presence of an Atheros 9331 processor (the bit under the metal can above) that runs concurrently with the AVR processor a Linux distribution based on OpenWrt called OpenWrt-Yun.
The board has built-in Ethernet and WiFi support, as well as a USB-A port, micro-SD card slot and 20 digital I/O pins of which 7 can be used as PWM outputs and 12 as analog inputs.
The AVR processor can communicate with the Linux distribution to leverage capabilities of that platform such as networking, shell and python scripts. The AVR side of the house has no need for a separate secondary processor as is found in the UNO to handle USB communications as it is built in.
The Yun device can appear to a connected computer as a mouse and keyboard in addition to a virtual serial / COM port.
A block diagramme can be seen below of the architecture. The bridge component facilitates communication between the two processors allowing sharing of abilities such as running shell scripts, access to network interfaces , USB host functionality and the SD card.
Out-of-the-box the Yun is WiFi enabled and it acts like an access point. You can connect your computer to the access point and then configure the Yun network. I joined it to my home WiFi network, rebooted it and now I can upload scripts to it over WiFi. Here is a shot of it sitting on top of a 10,000 ma/hr battery that is powering it as a stand alone unit that I can program from my laptop connecting via WiFi.
Here is a screen shot of an ssh session to the Linux side of the device and the normal Arduino IDE side-by-side.
It will be fun to see what new ideas will result from this enhanced functionality.