React is an Open Source JavaScript library  which is used for creating dynamic and interactive user interfaces for mobile and web applications.

It is highly flexible, declarative, and efficient for developing scalable, simple, and fast front-end for web and mobile applications.

React is only concerned with rendering data to the Document Object Model (DOM) and so creating React applications usually requires the use of additional libraries for implementing things like state management and routing.

DOM: Document Object Model is a way to represent the webpage in a structured hierarchical way. With DOM, we can easily access and manipulate tags, IDs, classes, Attributes, or Elements of HTML using commands or methods provided by the Document object. Using DOM, JavaScript gets access to HTML as well as CSS of the web page and can also add behavior to the HTML elements.

Virtual DOM: Virtual DOM is like a lightweight copy of the actual DOM. So for every object that exists in the original DOM, there is an object for that in React Virtual DOM. It is exactly the same, but it does not have the power to directly change the layout of the document.

Manipulating DOM is slow, but manipulating Virtual DOM is fast as nothing gets drawn on the screen.

Components: The component is the most basic building block of the React Application UI. 

One-way data-binding: React JS follows one-way data binding or unidirectional data flow that gives better control throughout the application.

JSX: React uses JSX for building templates instead of regular JavaScript. JSX is faster than normal JavaScript as it performs optimizations while translating to regular JavaScript.