D3, which stands for data-driven documents, is an open source JavaScript library used to create interactive web-based data visualizations. It provides a mechanism that connects arbitrary data to document elements, allowing their appearance and behavior to be driven by the data. Created by Mike Bostock, Jeff Heer, and Vadim Ogievetsky in 2001, it's currently used in hundreds of thousands of websites and is one of the most popular JavaScript data visualization libraries in the world.

If you have ever used interactive data applications from large news web portals such as The New York Times, The Washington Post, or The Guardian, there is a great probability that it was a D3 application. You may have also used one of the many charting libraries that are based on D3.

D3.js is also free and open source. You can use it in any project, commercial or not. Its source code is distributed in GitHub and is maintained by an active community of developers worldwide.

Yes, D3 is a JavaScript library, but no, D3 is not a charting library. There are no ready-to-use templates to create bar, pie, or line charts, for example. To create one of these charts, you have to draw all the lines, curves, and rectangles yourself using open standards such as SVG or HTML Canvas. D3, however, will do most of the hard work for you. It’s not trivial to use pure SVG to draw a bar chart; you need to scale data values so they fit in the chart, then calculate where to place each bar, and finally, set the coordinates of each rectangle before drawing it. Using D3, starting with an array of data, you can render all the bars with half a dozen chained commands in a single line of code.

D3 is a data visualization library. There are layout generators for pie charts that compute angles, which you can then use to draw arcs for the slices. There are functions that take a flat object array and turn it into a hierarchically linked object structure with coordinates for each node. You can use that data to draw circles at each coordinate point and draw lines between two nodes, rendering a tree. But you can also use the data differently, it's up to you. D3 doesn't restrict your creativity in any way. It doesn't tie you to a proprietary framework. Everything is based on open web standards.

D3 is also not only a data visualization library. Visualization is provided by HTML, CSS, or SVG. D3 focuses on the data. It's actually a collection of integrated JavaScript tools for manipulating the data structures necessary to create data visualizations. The core of the library is a fluent API used to select and manipulate the DOM. It replaces the DOM and libraries such as JQuery. It includes the data-driven mechanism that gives D3 its name, allowing you to bind arbitrary data to DOM elements, and then perform style and attribute transformations based on that data. This API is also used to bind and dispatch events, and to generate animated transitions.

D3 also includes tools to load and parse different data formats, such as JSON and CSV, perform general data manipulation on objects and arrays; generate data sequences and random numbers, perform interpolation and locale formatting. The actual data visualization parts contain layout generators, scales, axis generators, map projections, shape generators, color schemes, and other tools that are applied to previously selected DOM nodes and data.

A simplified view of D3's architecture is illustrated as follows. As implied by the name of the library, it's the data that drives the documents that display D3 visualizations. By adding, changing, and removing data, you directly affect the way your chart appears on the screen:

Data is usually provided as a JavaScript array, either generated locally or loaded from an external file. A typical D3.js script uses CSS selectors to select HTML or SVG elements and binds them to individual data items, removing, updating, or appending graphical elements automatically, when necessary.

You use CSS selectors to select one or more elements to create what D3 calls a selection object, which can then be used to apply styling and change the attributes of the selected elements. Binding an array to a selection object will map each data item to an element in the selection. These data items can be accessed in callback functions used in methods that set values for styles, attributes, and transforms.

You can also declare a selection of elements that don't exist in the DOM. Binding this selection to a data array will automatically create one element for each data item. You can then use the data to provide content, styles, and attributes for the new elements.

D3 also keeps data and their elements in sync. When updating with a smaller dataset, elements in excess are placed in a separate array, so you can remove them. If the dataset grows, existing elements are updated and missing elements are created and appended to fit the available data.