jQuery UI (User Interface) is an extension of the jQuery library aimed at simplifying the development of interactive and appealing user interfaces for web applications. It provides a collection of user-friendly widgets, effects, and interactions based on JavaScript and CSS.

Key features of jQuery UI include:

1. Widgets: jQuery UI contains various pre-built UI elements or widgets such as dialogs, buttons, progress bars, tabs, sliders, calendars, and more. These widgets are highly customizable and can be easily integrated into web pages.

2. Interactions: It offers functionality for implementing drag-and-drop features, sorting capabilities, resizing elements, and other interactive capabilities to enhance user experience.

3. Effects: Similar to jQuery, jQuery UI provides various effects and animations that can be applied to add, modify, or animate elements on the web page.

4. Theming: jQuery UI provides the ability to change or customize the appearance of widgets through theming. This means developers can adapt the look of the widgets to match the design of their website.

jQuery UI was developed to facilitate the creation of consistent and user-friendly user interfaces. It works closely with the jQuery library, extending its functionality with specific UI elements and interactions. However, with the advancement of CSS3 and the evolution of modern browsers, the use of pure CSS techniques or other UI development frameworks has increased in some cases compared to utilizing jQuery UI. Nevertheless, jQuery UI remains a relevant option for developers working on jQuery-based projects to create engaging user interfaces.