The Ethernet Shield for Arduino uses a W5100 chip to allow your Arduino to connect to the internet. To use the shield, mount it on top of an Arduino board. To upload sketches to the board, connect it to your computer with a USB cable as you normally would. Once the sketch has been uploaded, you can disconnect the board from your computer and power it with an external power supply. Connect the shield to your computer, a network hub or router using a standard Ethernet cable (CAT5 or CAT6 with RJ45 connectors).
The Wiznet W5100 provides a network (IP) stack capable of both TCP and UDP. It supports up to four simultaneous socket connections. Use the Ethernet library to write sketches which connect to the internet using the shield.
The board uses pins 2,10,11,12 and 13 for communication.