User Interface Designer sees an existing design and thinks, “Hey! Look at that! Let me make it pretty :-)”
User Interface designers spend most of their time making things look pretty. That is why you find most UI designers being highly active on design-centric networks like Dribbble, Instagram and Behance. Some would call UI young and vibey. It is a very showy environment, the ultimate status symbol being thousands of likes and followers. Criticsm is often met with reverse criticssm or ignored or politely acknowledged and then ignored.
User Experience Designer sees the same thing UI Designer saw and thinks, “I wonder a) How this design’s decisions were arrived upon, b) what was going through the mind of designer as they created this and finally c) it would be interesting to find out how will user research findings including post-design feedback will help increase the values of key performance indictors.”
User Experience designers spend most of their time making sure things work. That is why you find most UX Designers on discussion-centric networks like Medium, Quora and LinkedIn Groups where they engage in open debate with a non-“admiring” audience where criticsm is often welcome as it helps make things work better. Some would call UX mature & boring.