The Internet is a global network of interconnected computer systems that use the Internet Protocol Suite (TCP/IP) for communication. It connects billions of devices, enabling data exchange, communication, and access to services worldwide. The Internet has transformed communication, commerce, education, and entertainment but also brings concerns about privacy, security, and the digital divide.

HTML (Hypertext Markup Language) is the standard language for creating web pages and applications. It structures content using elements and tags, which browsers interpret to display web pages. HTML has evolved, with HTML5 as the current version, introducing semantic elements, better multimedia support, and improved form controls. HTML works with CSS for styling and JavaScript for interactivity, forming the core of modern web development.

CSS (Cascading Style Sheets) is a styling language that defines the presentation of HTML or XML documents. It separates design from content, providing control over layout, colors, and fonts. CSS uses selectors to target HTML elements and apply styles. It supports responsive design with media queries, allowing layouts to adapt to various screen sizes. Key concepts include the cascade, inheritance, and specificity, which determine how styles are applied. Modern CSS includes Flexbox and Grid for advanced layouts, as well as animations and transitions for dynamic interfaces.

JavaScript is a high-level, interpreted programming language essential for creating dynamic, interactive web pages. It supports object-oriented, imperative, and functional programming, and is used both client-side in browsers and server-side with Node.js. JavaScript features dynamic typing, first-class functions, and prototype-based object orientation. Widely used in web development, it remains one of the most popular languages, evolving through the ECMAScript standards.

React is an open-source JavaScript library for building user interfaces, especially for single-page applications. Developed by Facebook, it enables developers to create reusable UI components that update efficiently using a virtual DOM. React's unidirectional data flow and component-based architecture support modularity and reusability. Its ecosystem includes tools like Redux for state management and React Native for mobile apps. With its declarative syntax, efficient rendering, and strong community, React is a popular choice for modern web development.