Widget overview
Even if you are already familiar with widgets, take a moment to look at how they work in the WordPress manager under Appearance | Widgets. You know that they display content on the frontend of your site, usually in a sidebar, and they also have text and control forms that are displayed only when you view them inside the manager. If you put on your thinking cap, this should suggest to you at least two actions: an action that displays the content on the frontend, and an action that displays the form used to update the widget settings inside the manager. There are actually a total of four actions that determine the behavior of a standard widget, and you can think of these functions
as a unit because they all live together in a single widget object. In layman’s terms, there are four things that any widget can do. In programmatic terms, the WP_Widget object class has four functions that you may implement:
• The constructor: The constructor is the only function that you must implement. When you «construct» your widget, you give it a name, a description, and you defie what options it has. Its name is often __ construct() , but PHP still accepts the PHP 4 method of naming your constructor function using the name of the class.

• widget() : It displays content to users on the frontend.
• form() : It displays content to manager users on the backend, usually to
allow them to update the widget settings.
• update() : It prepares the updated widget settings for database storage.
Override this function if you require special form validation.
In order to make your widget actually work, you will need to tell WordPress about
it by registering it using the WordPress register_widget() function. If you
want to get a bit more information about the process, have a look at WordPress’
documentation here: http://codex.wordpress.org/Widgets_API.