React.js is a popular JavaScript library used for building user interfaces, especially for single-page applications. It allows developers to create reusable UI components, making it easier to manage and scale large web applications.

React.js uses a virtual DOM to efficiently update and render components. When a component's state changes, React compares the virtual DOM with the real DOM and only updates the parts that need to change, resulting in faster and more efficient rendering.

React components are reusable building blocks that make up a React application. They can be functional or class-based and are used to define sections of the UI, with each component handling its own state and behavior.

JSX (JavaScript XML) is a syntax extension for JavaScript that allows developers to write HTML-like code within JavaScript. It makes writing React components easier and more readable by combining HTML structure with JavaScript logic.

State in React.js is an object that holds data or variables that may change over time within a component. State is used to control how a component behaves and renders, and changes to the state trigger re-rendering of the component.

Props (short for "properties") are used to pass data from one component to another in React.js. Unlike state, props are immutable and are mainly used to make components more dynamic and reusable.