What is the difference between a constructor and a prototype or class?

Prototypes and classes are similar in what they do. In both cases, you have a template or blueprint that is used to create new objects of that type (for instance, a Dog class or prototype).

A constructor is simply a function defined inside of a class or prototype that is automatically called whenever a new object of the class or prototype is created. It is used to set the initial state of variables inside of the object.

In PHP (which uses classes), a class and its constructor would be defined like this:

<?php

class Dog
{
    public $name;

    public function __construct($name)
    {
        // constructor sets up object
        $this->name = $name;
    }
}

$dog = new Dog("Shep");
echo $dog->name; // prints out "Shep"

In JavaScript (which uses prototypes), a constructor would look like this:

var Dog = function(name)
{
    // constructor sets up object
    this.name = name;
}

var dog = new Dog("Shep"); // the 'new' is really important here
console.log(dog.name); // prints out "Shep" to the console

I've talked about prototypes and classes as if they're the same things here. While they do accomplish similar goals, you should know that they are really quite different in their behavior. The JavaScript courses here do a much better job of explaining their differences than I will, however. :)