People in the comment threads seem to be confusing jQuery UI with jQuery. jQuery UI is collection of interactions, effects, and widgets built on the more ubiquitous jQuery library.
jQuery UI was also tightly tied to Filament Group so it was a commercial product looking to capitalize on the jQuery name. That whole think was not so successful and jQuery UI was never that great. 5+ years ago it was bloated and buggy and it has not seen much development since.