What are stateful components?

If the behaviour of a component is dependent on the state of the component then it can be termed as stateful component. These stateful components are always class components and have a state that gets initialized in the constructor.

class App extends Component {
  constructor(props) {
    super(props);
    this.state = { count: 0 };
  }

  render() {
    // ...
  }
}

React 16.8 Update:

Hooks let you use state and other React features without writing classes.

The Equivalent Functional Component

import React, {useState} from 'react';

const App = (props) => {
  const [count, setCount] = useState(0);

  return (
    // JSX
  )
}