UI, User interface and UX, User Experience arte far from being the same thing. Where one (the interface) is the platform or frame around what you do, the other (experience) is your perception about what happens when you do.
Designing the user interface requires a lot of forethought an a good insight in how people see and experience the interface. The User experience is something you have to measure to get an idea about how well you did your work..
Stumbleupon has an article about this.