UI, or User Interface represents the look of a website. Design, colors, fonts, etc. UX, on the other hand, represents the entire experience that user has on a website. That includes UI and other important elements like page load time, usability and logical layout, are there any pop ups, content value, writing style (if it's a blog) and overall feel of the website. Whole point of UX is as its name suggests, User Experience. You want to make a pleasant and simple to use environment for users.