You've learned that a React component is a small, reusable chunk of code that is responsible for one job, which often involves rendering HTML.
Here's another fact about components: every component must come from a component class.
A component class is like a factory that creates components. If you have a component class, then you can use that class to produce as many components as you want.
To make a component class, you use one of the methods in the React library:
Look at the code in app.js. A lot it is still unfamiliar, but you can understand more than you could before!
On line 4, you know that
React.createClass's job is to create a component class, which is like a factory for building React components. You also know that
React.createClass is one of the methods on the object returned by
require('react'), on line 1.