D3.js is a powerful 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.js is its ability to manipulate HTML, SVG, and CSS, enabling the creation of visually appealing graphics. It supports a wide range of data formats and provides various tools for animation and interaction, making it a popular choice among data scientists and web developers.