A light weight, fast JavaScript library, rich in features which makes JavaScript programming easier. It facilitates DOM traversa and manipulation, event handling, animation and AJAX much simpler .
jQuery UI is a combination of a set of user interface interactions, effects, themes and widgets, which is built on top of jquery JS library.
jQuery UI Effects are the features provided by the jQuery library to add various animation effects to a web page. Also, there are custom effects available for showing or hiding elements to add some visual effects.
jQuery Widgets are specialized, platform independent, cross browser compatible, stateful plugins rich in features, with a full life cycle along with some methods and events, built on an extensible base called jQuery UI Widget Factory
A few jQuery UI Widgets are listed below.