That means the keys on a calculator that enables you input numbers and carryout calculations is a part of its user interface.
Another good example is your dstv remote. You see the keypad with numeric keys and the menu and volume control keys that enable you change channels, control the volume or mute tv entirely, all those constitute the user interface of the remote control.
In software applications, UI is known as Graphical User Interface (GUI). Since software applications are basically inside a hardware, they do not have physical keys that users can press to operate or navigate the software.
The software's interface is made up of graphical keys or icons instead of physical keys. The user can control and operate the software application using the software's graphic interface. The graphic interface includes icons like the menu bar, search box, navigation arrows, comment boxes, settings button, etc.
These are examples of common UI icons.
Now, a good user interface is expected to possess the following features:
1. It should be user friendly
User friendliness like I've stated earlier is the major aim of a user interface. Every user interface should be able to make the experience of using the device or software app pleasant and easy.
2. It should have familiar features
Every user interface both for hardware devices and software applications should have uniform features. This is so that the user does not have to relearn how to use the device or application. We recognize the power bottun of a device for instance when we see the usual symbol that represents it.
Most GUI have a similar icon for the menu bar and search box. The user expects to see these things when using a hardware device or software application.
3. It should be easy to use
I believe a good user interface should be easy to use. All the features should be easily recognizable so the user doesn't need to read a manual to figure it out. The features should have labels so the user can know what function they perform.
4. It should be responsive
The icons and keys of a user interface don't mean jack if they do not perform the function they are meant to. Imagine if you try sliding this page up and it doesn't move? Or you tap the menu icon at the top right corner and nothing pops up. It would make using this site quite difficult isn't it. This shows to a great extent how a responsive user interface contributes to the general User experience of a device or application.
Everybody likes things that appeal to them. A good user interface should have features that appeal to the users. A website's user interface for instance should have colours and a font style that it's users would find attractive. This would go a long way to improving the general User experience of the website.
Finally, a user interface should be clear. Users should be able to figure out the function of any feature and how to use it. It wouldn't do well for a device or software application if users get confused on how to use it.
User interface is a very important component of a hardware device or software application. In some cases, a hardware user interface is needed to operate a software user interface. A good example is the mouse of a computer used to control the user interface of a website online.
Now, for the features of a good user interface, not every user interface would possess all these features. But, it is expected that a good UI should possess at least clarity, usefulness and responsiveness.