What React is
The foundation for the main course project is Facebook's React framework. It offers a way to modularise your application into components. Each component can be passed properties from the parent, and it can also hold some state that can be changed. You never have to update anything when properties or state change; a component just needs a render function for the initial rendering, and then React will do some behind-the-scenes magic to performantly use that for updates as well!
There is an excellent getting started section on the main React homepage which we heartily recommend. Prior to going through that this video serves as a good first getting-to-know-React flirt:
To get started yourself we recommend the course demo app.
Using React in the course
The reason behind choosing React is that it is a tiny API and thus has a rather low threshold when compared to other frameworks such as Ember or Angular. Also, since this course focuses on code organisation and best practices, we don't want to use a framework which answers most such questions for you! And, finally - React is rather brilliant!