User interface is the part of the machine that handles the human-machine interaction. User interface design requires a good understanding of user needs. A "USER" is individuals like you and me who are the final adopters of the software the UI is being developed for. In order for the software or program to give a viable output, the user needs to feed in the desired input. This is only possible if the principles and logic of User Interface are followed. If no such provision is made for the user it will be very tough for the user to operate the system and whereby which the user will refrain from making the purchase. Before the commencement of any software application, the most important concept of user interface needs to be followed which is Functionality requirements gathering. Without knowing the end-goal and what functions need to be undertaken to achieve the desired goals, no software can be built/operated. This is knowledge of user interface design is necessary, companies which design and who develop programmes for their customers or other companies invest millions of dollars in User Interface Design.
User experience (UX) design is the process of creating designs for software that provide meaningful and relevant results. This involves the design of both a software program's usability and the pleasure consumers will derive by using it. It is also concerned with integrating with the software aspects such as branding, design, usability, and functionality.Products that provide great user experience (e.g., the iPhone) are thus designed with the idea of consumption or use in mind.
When first uses a software product, a user tends to forms a momentary impression of the software which evolves over time, typically as the software is used over a period of time. During this process, the user’s perception, action, motivation & responses integrate to form a memorable experience which is called “the user experience.” This process sparks emotional responses, which determine whether the experience towards the software will be considered positive or negative. What this means is that once you start using a software, say Windows OS and get used to the user experience that Windows OS has to offers, you expect or want the similar experience across multiple products as you form an emotional bond with it. Hence, if Windows OS changes the UX by removing the "START BAR" from the bottom, most users will demand it to be revived or leave the platform entirely.
UX designers who are aware of the process of the User Experience Design Process, seek to create and form the factors influencing the process deliberately. To do this, a UX designer will consider the Why, What, and How of product use. The Why involves the users’ motivations for adopting a product, whether they relate to a task they wish to perform with it, or to values and views associated with the ownership and use of the product. The What addresses the things people can do with a product—its functionality. Finally, the How relates to the design of functionality in an accessible and aesthetically pleasant way. UX design starts from the Why, then determines the What and finally the How in order to create products that users can form meaningful experiences with. This, as not a simple process and intense thought, needs to be paid by a good User Experience Designer due to which most companies are willing to pay a bombshell salary to hire talented User Experience and User Interface designers.