Arduino Ethernet Shield is a easy way to connect your Arduino UNO boards to internet. This Ethernet shield is based on Wiznet W5100 chip which provides network (IP) stack capable of both TCP and UDP. This W5100 based Shield supports up to four simultaneous socket connections. Use Arduino Ethernet Library to write sketches which connects your Arduino boards to the Internet using RJ45 cable via this shield. This latest version Ethernet shield has on board micro-SD card slot, which can be used to store files for serving over the network. This latest Ethernet shield is compatible with Arduino UNO and MEGA boards using the Ethernet library and micro-SD is accessible by using SD library.
This Ethernet shield also includes a reset controller, to ensure that the W5100 Ethernet shield is properly reset on power up. The reset button the shield resets both the W5100 shield and Arduino board.
The Ethernet shield and micro-SD slot both uses same SPI bus for communication with the Arduino board, only one can be active at a time, this can be handled in your program.