All objects have a prototype property. It is simply an object from which other objects can inherit properties. The snippet you have posted simply assigns an object with some properties (such as init ) to the prototype of jQuery , and aliases jQuery.prototype to jQuery.fn because fn is shorter and quicker to type. If you forget about jQuery temporarily, consider this simple example: function Person ( name ) { this . name = name ; } Person . prototype . sayHello = function () { alert ( this . name + " says hello" ); }; var james = new Person ( "James" ); james . sayHello (); // Alerts "James says hello" In this example, Person is a constructor function. It can be instantiated by calling it with the new operator. Inside the constructor, the this keyword refers to the instance, so every instance has its own name property. The prototype of Person is shared between all instances. So all instances of P