The User Interface or UI is a subcategory of the human-machine interface and plays an important role in an Operating System project. The term "UI" describes the interaction of the OS or rather of applications with the user. The other direction, the interaction of the user with the OS or rather application is another subcategory of the human-machine interface. Nowadays this interface is visual, also some OS's support haptic, e.g. vibration, or auditory, f.e. for notifications, feedback. This interface will form the first impression of the user and based on it the user will decide either to use the corresponding OS or switch to another one.
There are two main types of UI's: text user interfaces, often called command line interfaces, and graphical user interfaces. Text interfaces are easier to implement, while graphical user interfaces are easier to use, espacially for beginners.