'User experience' has become such a buzzword. I have noticed when some of my clients talk about UX, what they really mean is UI design.
The term 'user experience' was coined by Don Norman - “I thought human interface and usability were too narrow. I wanted to cover all aspects of the person’s experience with the system, including industrial design graphics, the interface, the physical interaction, and the manual.”
User experience is any aspect of a person's interaction with a product, service or organisation. A post from Smashing-magazine gives a very nice UX intro.