What is Iterator and collections? Does these two have any relations?

// the interface definition
Interface Iterator {
boolean hasNext();
Object next(); // note "one-way" traffic
void remove();
}

// an example
public static void main (String[] args){
ArrayList cars = new ArrayList();
for (int i = 0; i < 12; i++)
cars.add (new Car());
Iterator it = cats.iterator();
while (it.hasNext())
System.out.println ((Car)it.next());
}

Does the Interface Iterator has these method names alone predefined or its user defined?. What does these four lines below actually tell?

cars.add (new Car());
Iterator it = cats.iterator();
while (it.hasNext())
System.out.println ((Car)it.next());

Thanks i am going through a book in collections.