What’s the purpose of prototype? [duplicate]

Possible Duplicate:
Use of ‘prototype’ vs. ‘this’ in JavaScript?

OK, So I am somewhat new to the idea of OOP in JS.

What is the difference between these two snippets of code written below:

function animal(){
    this.name="rover";
    this.set_name = function(name){
         this.name = name;
    }
}

function animal(){
    this.name="rover";
}
animal.prototype.set_name = function(name){
    this.name = name;
}

They both do the same thing, so what’s the difference?