Ethernet Shield

The ethernet shield allows basic I/O over an TCP/IP network.

Using the SD slot

If you want to use the sdfatlib library, you absolutely need the following lines in your setup to access the SD card.

// set the SS pin as an output (necessary!)
pinMode(4, OUTPUT);               
 // turn off the W5100 chip!        
digitalWrite(4, HIGH);                    
// use digital 4 as the SD SS line
uint8_t r = card.init(SPI_HALF_SPEED, 4);

For more detailed information about this, please read the official Arduino Ethernet + SD page.

Links

 Ethernet Shield

 Arduino Ethernet + SD

 sdfatlib homepage

 How to use the SD card

Data exchange

When exchanging data with other system, think about JSON. Your Arduino could output somthing like this:

{"temperature":22.70,"pressure":1002.71,"humdity":48.86,"light":91.50}

Using PHP, to only state an example, you could then get and decode the data like this:

$data = json_decode(file_get_contents('http://<ip_of_your_arduino>'));