WordPress Widgets are used for adding the features and content to your Sidebars. The example shows default widgets for tag clouds, navigation, post categories, search etc., which are delivered with WordPress. Plugins, however, often introduce widgets of their own.
The initial purpose of widgets is to provide the user with a user-friendly and simple way to control the design and structure of the WordPress Theme. Nowadays, this feature is common among properly “widgetized” WordPress Themes. This allows the users to include widgets in the footer, header or otherwhere in the WordPress design and structure.