jQuery UI is a curated set of user interface interactions, effects, widgets, and themes built on top of the jQuery JavaScript Library. Whether you're building highly interactive web applications or you just need to add a date picker to a form control, jQuery UI is the perfect choice.
jQuery UI contains many widgets that maintain state and therefore have a slightly different usage pattern than typical jQuery plugins. All of jQuery UI's widgets use the same patterns, so if you learn how to use one, then you'll know how to use all of them.