If you're new to React, this is a good place to start. This guide contains everything you need to learn to get started with React along with interactive Sandbox's for you to practice.
Title says it all. This article walks through how the internals of React tells the difference between a Class and a Function (component).
Have you ever wondered why you have to write
super(props) when you're using a Class component with a
constructor method? Wonder no more.