Well, I always think of a User Experience Designer as someone who can construct site architecture, understand content and create high-level conceptual wire frames down to detailed interaction design wire frames.
A Web Designer/ Visual Designer (to me) might be someone who focuses more on the visual and developmental aspects of web design (photoshop/flash/css). However, I’d prefer to use the distinction of Interactive designer. It’s clearer that it’s someone who designs for the digital space but doesn’t necessarily code (as opposed to a developer)
Here’s my tip on this subject. If you’re a Visual Designer than it’s pretty easy to make a portfolio you really just need to showcase your work.
But as a User Experience Designer I think it’s important to not only show the work, but to explain the process and logic that went into the design. Show the sketches and wire frames along side the finished design. Also explain the demographics/personas you may have been targeting as well as the overall goals of the project.
But what i you do both kind of work?
I think the same way as a User Experience Designer. Just with more of a focus on the finished design. So have your main image and thumbnails be the finished design.