The Widgets can be added or changed from the ‘Widget Panel’ in the ‘Appearance’ section of WordPress ‘Administration’. Default Widgets present in the WordPress Sub-Panel are post categories, tag clouds, navigation, search, etc. Some Plugins also add widgets to the sidebars. The Widgets can be used differentially with different Themes, Plugins, and in case of need they can be developed for customized use.
Each Widget has its own benefit and adds new feature or functionality to WordPress. Widget offers good customization methods that fit your need by either adding a single or multiple Widgets of the same kind (Text, RSS, or other Widgets) to widget placeholder sections of website, or blog, or page. To do this, you either have to use already present WordPress Widgets or install new Widgets. Once available for use in the Widget Section (Administration > Appearance > Widget), you can drag and drop the Widgets wherever you need in page’s Widget Placeholder. Widgets (Widget Plugins) in the Category Widgets in Plugin download section allows for new features and functions to WordPress. You can just download, install, and activate the Widget as a new Plugin Install.
Many Widgets like Text or Custom Menu have to be setup, or configured (using Widget settings), or personalized for your website/ blog; once they have been added to the page’s Widget Placeholder. When using specific Themes, Widgetizable Themes use and customization changes how your blog or site looks. Most Themes do offer them. New Themes with Widget capability can be installed in ‘Themes’ Menu and once installed, Widgets can be added to their Widget placeholders to customize or to improve the page layout or look.
Widget Development is one of the important aspects of WordPress. WordPress Widget Development is a subset of Plugin development and uses WordPress Widget API for development. Development is not much different than Plugin or Theme development, other than specific use of Widget API, or Theme based and callable Widget API. For example: In case of Widgetized Themes, you have to use the development Theme functions of the Widget Development API, and then is developed as a Theme add-on. In case of Widget Plugin, you develop the Widget using the Widget API as a Plugin to be easily installed.
Technically a widget is a PHP object that echoes string data to STDOUT when its widget() method is called. The WordPress Widget API is located in wp-includes/widgets.php. Help and support for the WordPress API can be found in the Widget API section.
WordPress Widgets: http://codex.wordpress.org/WordPress_Widgets
WordPress Widget Sub-Panel: http://codex.wordpress.org/Widgets_SubPanel
WordPress Widgetizing Themes: http://codex.wordpress.org/Widgetizing_Themes
Widget API: http://codex.wordpress.org/Widgets_API
External Site – Designing a Theme to Include Widgets: http://lorelle.wordpress.com/2006/03/31/designing-a-theme-to-include-wordpress-widgets/
External Site – WordPress and Development API Documentation: http://automattic.com/code/widgets/api/