This Ethernet Shield is based on the Wiznet W5100 Ethernet Chip and gives you an easy way to get your Arduino online. It is directly supported by the official Arduino Ethernet Library. It also includes an additional micro-SD card slot, which can be used to store files for serving over the network. It is compatible with the Arduino Duemilanove (168 or 328), Uno as well as Mega (1280/2560) and can be accessed using the SD and Ethernet libraries.
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.
Example Arduino code available on our support forum.