URD is a program to download binaries from usenet (newsgroups) through a web interface.
While some other programs offer similar services (e.g. Ninan, Sabnzbd), URD doesn't need external NZB files to function. URD lets you subscribe to newsgroups, and will then automatically analyse those articles to form downloadable 'sets', consisting of all the files belonging to a specific upload. These sets are displayed, and through point and click you can be downloading a binary in no time.
That isn't to say that NZB files are unsupported. URD allows both exporting of NZB files (from a set, for example) and importing. And it can use RSS feeds to download from directly.