The UI (User Interface) is a concept that encompasses information architecture, patterns, and different visual elements that allow us to interact effectively with operating systems and software on various devices.
Imperative vs Declarative Programming
Declarative programming is a paradigm that describes WHAT the program does, without explicitly specifying its flow of control.
By contrast, imperative programming is a paradigm that describes HOW the program should do something by explicitly specifying each instruction (or statement) step by step, which mutates the state of the program.
The easiest way to explain the difference between declarative and imperative code would be that imperative code focuses on writing an explicit sequence of commands to describe how you want the computer to do things, and declarative code focuses on specifying the result of what you want.
Single Page Applications (SPAs)
To answer the question, What is React? We must talk about a new concept, SPA.
A Single Page Application (SPA) is a web application that is presented to the user through a single HTML page to make it more responsive and more closely replicate a desktop application or a native application. A SPA is sometimes called a Single Page Interface (SPI).
You will easily recognize some popular examples of single-page applications such as Gmail, Google Maps, Airbnb, Netflix, Pinterest, Paypal, and many more. A lot of companies use SPA to create a seamless and scalable experience.