What are Components in ReactJS ?

React Components let you split the UI into independent, reusable pieces, and think about each piece in isolation.

Conceptually, components are like JavaScript functions. They accept arbitrary inputs (called “props”) and return React elements describing what should appear on the screen.

Below is ample component written in ES6 class to display a welcome message on the screen.

class Welcome extends React.Component {
  render() {
    return <h1>Hello, {this.props.name} </h1>;
  }
}

const element = <Welcome name="Sara" />;
ReactDOM.render(
  element,
  document.getElementById('root')
);

source: https://facebook.github.io/react/docs/components-and-props.html