It connects your Arduino to the internet wirelessly; all you have to do is to follow simple instructions in order to connect it to your wireless network. Also, did you know that EVERY element of the platform (hardware, software, & documentation) is available also the open-source, which means that you can actually start controlling your world through the internet! If you want a starting point for YOUR OWN circuits, you can learn how it’s made and benefit from its design, and if you want to understand how the shield provides internet connection to your Arduino board then you should definitely check out the reference page of WiFi library.
The WiFi Shield communicates with Arduino using the SPI bus (through ICSP header), so it’s compatible with any of the boards that have this type of bus. This version of WiFi shield have an integrated antenna on the PCB, Just connect the shield to the Arduino.