What we do here is that we create a function, called DocumentReady, which should be fired as soon as the document is ready for DOM manipulation. In the last line, we use the ready() method to assign our function to the ready event, to tell jQuery that as soon as the document is ready, we want it to call our function.
But of course, this wasn't even short enough for the jQuery team, so they decided to create a version (overload) of the jQuery constructor which takes a ready function as a parameter, to make it even shorter:
In the last example, our anonymous function is passed directly to the jQuery constructor, which assigns it to the ready event. As you will see when you test the code, the event is fired as soon as the page is loaded, most of the time so fast that you won't even realize it.
As already described, wrapping your code in the ready event function is best practice for working with jQuery in your document, and therefore you will see this tutorial using the approach in most of the examples, unless skipped to keep example sizes down.