Ethernet Shield is based on the Wiznet W5100 ethernet chip providing a network (IP) stack capable of both TCP and UDP. The Arduino Ethernet Shield 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.
The latest 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 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 build.
The latest revision of the shield also includes a reset controller, to ensure that the W5100 Ethernet module is properly reset on power-up. Previous revisions of the shield were not compatible with the Mega and need to be manually reset after power-up. The reset button on the shield resets both the W5100.