We found this article instructive and clear in its discussion of how user experience (UX) combines with user interface (UI) and graphic design (GD) to provide a complementary whole to a website's useability.
A visible example is provided by the complete design of the Apple experience which extends through products, apps, the shop and even the operating system.
The article is focussed on developing design careers, however we like its details and ease of reading.
You don't have to be a web designer to need this information - It can be useful in any understanding of how a web presence works for any business. Happy reading!