Arduino Ethernet Shield W5100 is one of the most interesting expansion boards allows to control the Arduino board via LAN or Internet. This can be useful, if you need to get some information from a network, place your own web server. Or use Arduino for control a network.
On this Ethernet Shield W5100 also has the microSD card socket.
For the control of board using the base library Arduino IDE - Ethernet library, and for microSD cards - SD library.
For addition to the Arduino board and this shield, you will also need a network cable (twisted pair).