One type of UI (user interface) is graphical (GUI) which provides the user with a visible interface, making a computer a lot easier to use. This means you can use peripherals such as a mouse and keyboard to manage windows and menus.
Another common type of user interface, is menu driven. This is mostly used on less complex computers such as ATM machines, music players, washing machines etc. This is used with buttons and is simpler than a graphical user interface.
The third main type of UI is command line. This uses no graphics and is purely text. It's used mainly by experienced users who understand the commands. These interfaces only input and output text which means they use very little RAM.