User Interface vs. User Experience
When you talk about interface design, you end up using these terms (UX and UI) a lot. Of the two, UX seems to be the more fashionable and I hear it (and probably say) a lot more than UI. Let's face it, user experiencecarries an air that user interfacedoesn't. UI is so 1990, so toolbar. UX is much more ribbon.
The truth, however, is that the two are not interchangable and if you really want look smart, wikipedia can help you sort out the difference. There's also a nice history of the term UX in this article where you'll find out that the term really finally took off with help from Don Norman while he was working at Apple. Nice one UX!
The key point is that UX has a much large scope than UI as it refers to much more than just the interface. It's the whole ball of wax: everything from brand perception to hardware design to customer support calls get crammed into that tiny term. It's everything the user experiences and it's a lot to take on.
So if you're just talking about what's on the screen, UI might be a safer term. On the other hand, the power of the X in UX is in remembering that UI is just one component of something much bigger.