JSX (JavaScript XML) is a syntax extension for JavaScript that allows developers to write HTML-like code within JavaScript. It improves readability by combining UI structure with dynamic logic. JSX elements must be properly closed, use camelCase attributes, and can embed JavaScript expressions inside curly braces. For example,

Hello, World!

React elements are the building blocks of UI and can be rendered to the DOM using ReactDOM.render(). React efficiently updates the necessary elements when data changes. Elements are immutable, so to update the UI, new elements must be created and rendered. Using JSX and ReactDOM.render() together helps create dynamic, interactive web applications with optimized performance.