digitalRead()

Reads the value from a specified digital pin, either HIGH or LOW.

digitalRead(pin)

pin: the number of the digital pin you want to read (int)

HIGH or LOW

int ledPin = 13; // LED connected to digital pin 13
int inPin = 7;   // pushbutton connected to digital pin 7
int val = 0;     // variable to store the read value
 
void setup()
{
  pinMode(ledPin, OUTPUT);      // sets the digital pin 13 as output
  pinMode(inPin, INPUT);      // sets the digital pin 7 as input
}
 
void loop()
{
  val = digitalRead(inPin);   // read the input pin
  digitalWrite(ledPin, val);    // sets the LED to the button's value
}

Sets pin 13 to the same value as the pin 7, which is an input.

If the pin isn't connected to anything, digitalRead() can return either HIGH or LOW (and this can change randomly).

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

* pinMode() * digitalWrite()

Source: arduino.cc