jQuery $(document).ready() is a basic part of using jQuery. Document ready event is a self-executing function which fires after the page elements have loaded. jQuery document ready is used to initialize jQuery/JavaScript code after the DOM is ready, and is used most times when working with jQuery.

There are different ways of writing Document Ready functions typically used in jQuery:-

Example 1:

$(document).ready(function() {
// Put javascript here
});

Example 2:

 $(function(){ 
    // put jQuery code here 
    });

The above two codes are literally similar.

Example 3:

jQuery(document).ready(function($) {
// put jQuery code here 
});

Adding the jQuery can help prevent conflicts with other JS frameworks.

Conflicts arises because many JavaScript Libraries/Frameworks use the same shortcut name which is the dollar symbol $,hence the browser gets confused.To prevent conflicts jQuery namespace is recommended,then you call $.noConflict() to avoid namespace difficulties.

jQuery.noConflict(); // Reverts '$' variable back to other JavaScript libraries, avoiding namespace difficulties
jQuery(document).ready( function(){
   // Put javascript here when DOM is ready with no conflicts
});

Example 4:

(function($) { 
// code using $ as alias to jQuery
$(function() {
// more code using $ as alias to jQuery
});
})(jQuery);
// other code using $ as an alias to the other library

This way you can embed a function inside a function that both use the $ as a jQuery alias.

Example 5:

$(window).load(function(){
//initialize after images are loaded
});

if you want to execute code when the HTML doc is completely loaded,i.e,the DOM and its content,then this method is used.