D3.js (Data-Driven Documents) is a JavaScript library for creating interactive and data-driven visualizations in web browsers. It uses HTML, SVG, and CSS to render charts, graphs, and complex visualizations based on data inputs. D3.js offers fine-grained control over visual elements, allowing developers to create highly customized and dynamic visual experiences. It is widely used in data analytics, business intelligence dashboards, and academic projects. D3.js is favored for its flexibility and ability to handle large datasets effectively.