pinMode()

Configures the specified pin to behave either as an input or an output.

As of Arduino 1.0.1, it is possible to enable the internal pullup resistors with the mode INPUT_PULLUP. Additionally, the INPUT mode explicitly disables the internal pullups.

pinMode(pin, mode)

pin: the number of the pin whose mode you wish to set

mode: INPUT, OUTPUT, or INPUT_PULLUP.

None

 
int ledPin = 13;                 // LED connected to digital pin 13
 
void setup()
{
  pinMode(ledPin, OUTPUT);      // sets the digital pin as output
}
 
void loop()
{
  digitalWrite(ledPin, HIGH);   // sets the LED on
  delay(1000);                  // waits for a second
  digitalWrite(ledPin, LOW);    // sets the LED off
  delay(1000);                  // waits for a second
}

The analog input pins can be used as digital pins, referred to as A0, A1, etc.

* constants * digitalWrite() * digitalRead() Source: arduino.cc