This Ethernet Shield allows an Arduino board to connect to the internet. It is based on the Wiznet W5100 ethernet chip providing 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 via a standard RJ45 Ethernet jack using the shield.
This revision of the shield adds a micro-SD card slot, which can be used to store files for serving over the network. It is compatible with the Arduino Uno and Mega (using the Ethernet library). You can access the on-board SD card slot using the SD library which is included in the current Arduino build.
Arduino Ethernet Library