Comparator Interface Implementation?

Something about implementing Comparator interface isn't very clear to me: overriding the compare method.

Like here for example:

//This sorts a list of objects holding information based on age: the name and the age of the person
 
public class Person {
String name;
int age;
public Person (String name, int age)
{
this.name = name;
this.age = age;

[Code] ....

What exactly is happening behind the scenes? I don't understand mostly the part where it returns a 0, a 1, or a -1. After it returns one of those values, what really happens next?

For the displaying of the list, is the method toString() being accessed to output the list in the System.out.println statement?

For the generics, why do we use Person?