D3 is a JavaScript library used for creating dynamic and interactive data visualizations in web browsers. It allows developers to bind data to Document Object Model (DOM) elements and apply data-driven transformations to the document. This makes it easier to create complex visual representations of data, such as charts, graphs, and maps. One of the key features of D3 is its ability to manipulate the visual elements based on data changes, enabling real-time updates. It supports various data formats, including CSV, JSON, and XML, making it versatile for different types of data visualization projects.