The Arduino Ethernet is discontinued now. We recommend many other Arduino Microcontrollers as suitable replacements where you might find something interesting.
The Arduino Ethernet is a microcontroller board based on the ATmega328. It has 14 digital input/output pins, 6 analog inputs, a 16 MHz crystal oscillator, a RJ45 connection, a power jack, an ICSP header, and a reset button.
NB: Pins 10, 11, 12 and 13 are reserved for interfacing with the Ethernet module and should not be used otherwise. This reduces the number of available pins to 9, with 4 available as PWM outputs.
An optional Power over Ethernet module can be added to the board as well.
The Ethernet differs from other boards in that it does not have an onboard USB-to-serial driver chip, but has a Wiznet Ethernet interface. This is the same interface found on the Ethernet shield.
An onboard microSD card reader, which can be used to store files for serving over the network, is accessible through the SD Library. Pin 10 is reserved for the Wiznet interface, SS for the SD card is on Pin 4.
A six pin header can be connected to an FTDI breakout board or USB Serial board to provide USB power and communication to the board.

Specifications:

• Microcontroller: ATmega328
• Operating Voltage:5V
• Input Voltage (recommended):7-12V
• Input Voltage (limits):6-20V
• Digital I/O Pins:14 (of which 4 provide PWM output)
• Arduino Pins reserved
• 10 to 13 used for SPI
• 4 used for SD card
• 2 W5100 interrupt (when bridged)
• Analog Input Pins6
• DC Current per I/O Pin:40 mA
• DC Current for 3.3V Pin:50 mA
• Flash Memory32 KB (ATmega328) of which 0.5 KB used by bootloader
• SRAM2 KB (ATmega328)
• EEPROM1 KB (ATmega328)
• Clock Speed16 MHz
• W5100 TCP/IP Embedded Ethernet Controller
• Power Over Ethernet ready Magnetic Jack
• Micro SD card, with active voltage translators