React.createClass is the way to make a new component class.
When you make a new component class, you should store it in a variable so that you can use it later. On line 4, notice that our component class is stored in a variable named
Component class variable names must begin with capital letters!
This adheres to the naming convention in which class names are written in UpperCamelCase. There are technical reasons for it as well. We'll get to those soon!