User experience (UX) design is a field responsible for the overall flows and interactions of a product, attempting to minimize friction for users and make sure that the resulting product is easy to use and provides a great user experience (eg., the iPhone). While it can be applied to nearly any user-facing industry, it is most used in the tech industry with regards to work on user-facing software applications.
The terms “user interface design” and “usability” are often used interchangeably with “UX design”, however, user interface design and usability are really just subsets of UX design. UX design encompasses the entire journey and process of obtaining, owning, troubleshooting, and putting together a product (even before it's in the user's hands), which includes design, branding, function, and usability. UX design also focuses on creating a product that is not only efficient and seamless, but fun to use.
When looking to create a product with great user experience, UX designers need to remember the what, why, and how of a product's use. There is never just one concrete definition of what a great user experience is, or does. Therefore, being flexible and focusing on 'what' the user's needs are, 'why' they would want to use the product, and 'how' they need the product to perform, is key to creating something both useful and meaningful.
Because UX design is 100% user-centred, it is multidisciplinary. It has to bring together a variety of components in order to be successful at championing for the user. Some of these components include user accessibility, great interaction design, an understanding of human psychology, and an empathy towards physical limitations that users may have. Keeping the user's needs at the centre of all design and development efforts helps to address all relevant user needs and issues in the best way possible.