What is D3.js?

The fine-grained control and its elegance make D3 one of the most powerful open source visualization libraries out there. This also means that it's not very suitable for simple jobs, such as drawing a line chart or two--in that case, you may want to use a library designed for charting. Many use D3 internally anyway. For a massive list, visit https://github.com/sorrycc/awesome-javascript#data-visualization.

D3 is ultimately based around functional programming principles, which is currently experiencing a renaissance in the JavaScript community. This book isn't really about functional programming, but a lot of what we'll do will seem really familiar if you've ever used functional programming principles before. If you haven't, or come from an Object-Oriented (OO) background like I do, don't worry, I'll explain the important bits as we get to them, and the revised section on functional programming at the beginning of Chapter 4, Making Data Useful, will hopefully give you some insight into why this paradigm is so useful, especially for data visualization and application construction.

What happened to all the classes?

The second edition of this book contained quite a number of examples using the class feature that is new in ES2015. The revised examples in this edition use factory functions instead, and the class keyword never appears. Why is this, exactly?

ES2015 classes are essentially just syntactic sugaring for factory functions. By this I mean that they ultimately transpile down to factory functions anyway. Although classes can provide a certain level of organization to a complex piece of code, they ultimately hide what is going on underneath it all. Not only that, using OO paradigms, such as classes, is effectively avoiding one of the most powerful and elegant aspects of JavaScript as a language, which is its focus on first-class functions and objects. Your code will be simpler and more elegant using functional paradigms than OO, and you'll find it less difficult to read examples in the D3 community, which almost never use classes.

There are many, much more comprehensive arguments against using classes than I'm able to make here. For one of the best, refer to Eric Elliott's excellent The Two Pillars of JavaScript pieces at:
www.medium.com/javascript-scene/the-two-pillars-of-javascript-ee6f3281e7f3.