React is a very popular choice when building a SPA, Single Page Application.
A Single Page Application differs from a normal web application in that you remain on the same URL and thereby the same page, hence the name. To create many pages in a Single Page Application we instead use the hash, # and listen to changes on it, for example http://myapp.com#/products and http://myapp.com#/users is considered different pages in a Single Page Application. More on routing in the Routing section.
React was created by Facebook.
The point of this book is to try to cover what I've learnt and hopefully make other people better in the process. Everyone benefits from us being better at using something.
If you are new to React spend some time in the basic section before moving on. It's important to know your basics ( I've learned this lesson the hard way :) )
The official documentation can be found at, https://reactjs.org/
Questions or comments, you can reach me on twitter: @chris_noring
Happy reading Chris