User Interface (UI) refers to the look of the website. It serves as the window of your store online. Users, at the first glance, would only see the look of the website, if it attracts them, they will continue surfing. So, the importance of a good UI would be mainly in attracting users to continue engaging with your message, learning about your services, and/or searching for your products. User Experience (UX) on the other hand, would refer to the experience a user will have while visiting your website. A good UX will convey the message in a clean and comprehensive way.
The flow of the website is straight forward and the users will be led to the important parts, which the business needs them to land to. The visitors of the website will not get lost with loads of information, instead, they will be guided throughout the website.
Since professionally created websites look attractive, easy-to-use and handy to the visitors, the first time visitors often will be converted to returning customers of the business.