shows the program with and without using an interrupt

int.cpp

/*
  This program just toggles an led when you press a button
 */

// without interrupt

#define LED 13
#define BUTTON 2

boolean last = LOW;
boolean ledOn = LOW;

void setup() {
  pinMode(LED, OUTPUT);
  pinMode(BUTTON, INPUT);
}

void loop() {
  boolean current = digitalRead(BUTTON);
  if (current == HIGH && last == LOW) ledOn = !ledOn;
  last = current;
  digitalWrite(LED, ledOn);
}

// with interrupt...
// gets rid of a global var, a local var, an if, and an assignment

#define LED 13
#define BUTTON 0 // Intr 0 == Pin 2

volatile boolean ledOn = LOW;

void setup() {
  pinMode(LED, OUTPUT);
  attachInterrupt(BUTTON, toggle, RISING);
}

void loop() {
  digitalWrite(LED, ledOn);
}

void toggle() {
  ledOn = !ledOn;
}