Ethernet Shield is an ethernet shield which is prepared and manufactured on a circuit schema which is provided by Arduino. It is compatible with Original Arduino Ethernet Shield and all libraries and codes which are made for Original Arduino Ethernet Shield can be used with it.
You can connect your Arduino board to internet in a few minutes by using Ethernet Shield. Stack Ethernet Shield to Arduino and RJ45 cable to ethernet shield, after following couple of steps you can start controlling Arduino through internet.
Ethernet Shield provides internet connection through ethernet. This board which is based on Wiznet W5100 IC is compatible with both TCP and UCP. Ethernet Shield supoorts 4 socket connections at the same time. You can connect Arduino to internet fast with Ethernet Library by connecting ethernet cable to standard RJ45 ethernet socket on board.
Ethernet Shield has also a version with PoE. You can check Ethernet Shield with Poe.
There is a SD card socket on board. Therefore you can write various information to or read them from SD card. SD card can be used with SD Library. SD card's SS pin is; pin 4.
Arduino, communicates with Wiznet 5100 which is the processor of Ethernet Shield and SD card through SPI line. Those pins are 11, 12 and 13 on UNO and 50, 51 and 52 on MEGA. 10th pin is W5100's and 4th pin is SD card's select pin on both models. These pins cannot be used as general input or output. There is no hardware SS pin (53rd pin) for SD card or W5100 on Arduino Mega. But this pin must be set as output pin.
The important point that sould not be forgotten is since W5500 and SD card use same SPI line, both of them cannot be active at the same time, only one can. If two communications are wanted to be at the same time, library must be mutual. If you do not use both of them at the same time the you must deactivate one. To inactivate SD card you need to set 4th pin as output and give 5V out (HIGH) through this pin. To inactivate W5500 you need to set 10th pin as output and give 5V out (HIGH) through this pin.
P.S: For combined version of Ethernet Shield and Uno you can check Ethernet.