analogRead

int analogRead(uint8_t pin);

This function reads the current voltage from an analoy pin, and returns 0 for 0v, 1024 for 5v, or somewhere inbetween for other voltages relative to the maximum. There are five analog pins. If you read a pin that is not connected to anything, the function will return nothing useful.

Usage:

//I want to know Pin 2's value
int pin2Value = analogRead(2);

//this is about the voltage read (warning: floating point operatinos are slow!)
float pin2Voltage = pin2Value / 1024.0;