User Experience (UX) is a term that refers to how users of products and services experience the usage of these products and services. It therefore encompasses a much wider domain than the related fields of interaction design and service design. There have been a couple of articles in blogs and other online media that discuss the domain of UX:

Helge Fredheim: http://www.smashingmagazine.com/2011/03/15/why-user-experience-cannot-be...

Oliver Reichenstein: http://www.informationarchitects.jp/en/can-experience-be-designed-2/

Dane Petersen: http://www.adaptivepath.com/ideas/can-experience-be-designed

Jodi Forlizzi: http://interactions.acm.org/content/?p=1397