UI (User Interface) is how a website looks. UX (User Experience) is how it works and feels to use. Both matter, but UX — the experience of using the site — has the greater direct impact on conversions.
UI stands for User Interface — the visual layer of a website or application: buttons, typography, color, layout, icons, and every visual element a user sees. UX stands for User Experience — the totality of how a person feels using the site: whether they can find what they need, whether the flow is intuitive, whether the interactions work as expected, and whether they achieve their goal without frustration. The two disciplines are related but distinct.
A useful way to understand the difference: UI design determines whether a site is beautiful. UX design determines whether it's useful. A beautiful website with confusing navigation has strong UI and poor UX. A plain website that efficiently guides visitors to contact the business has poor UI but strong UX. For business websites where the goal is lead generation, UX has the higher direct impact on results.
Good UX for a service business website means: visitors understand what you do and who you serve within 5 seconds of landing; navigation is intuitive and doesn't require trial and error; the path from arrival to contact is obvious and friction-free; mobile users can accomplish everything desktop users can; and pages load fast enough that users don't give up waiting.
UX design is often mischaracterized as purely aesthetic or subjective. In practice, many UX decisions are measurable and testable — conversion rate, time on page, scroll depth, and form completion rates all reflect UX quality. A UX problem is a business problem. Fixing navigation confusion, contact form friction, or page clarity issues directly increases revenue without requiring more traffic.