Element Vs Component in React js

An Element is a plain object describing what you want to appear on the screen in terms of the DOM nodes or other components. Elements can contain other Elements in their props. Creating a React element is cheap. Once an element is created, it is never mutated.

the object representation of React Element.

const element = React.createElement(
  'div',
  {id: 'login-btn'},
  'Login'
)

The above React.createElement() function returns an object:

{
  type: 'div',
  props: {
    children: 'Login',
    id: 'login-btn'
  }
}

And finally it renders to the DOM using ReactDOM.render():

<div id='login-btn'>Login</div>

Whereas a component can be declared in several different ways. It can be a class with a render() method or it can be defined as a function. In either case, it takes props as an input, and returns a JSX tree as the output:

const Button = ({ onLogin }) =>
  <div id={'login-btn'} onClick={onLogin}>Login</div>

Then JSX gets transpiled to a React.createElement() function tree:

const Button = ({ onLogin }) => React.createElement(
  'div',
  { id: 'login-btn', onClick: onLogin },
  'Login'
)

React Element

A React Element is an object representation of a DOM node. It is a plain old object, and not an actual DOM element.

{
  type: 'h2',
  props: {
    className: 'heading heading-medium',
    children: 'React Deep Dive'
  }
}

const header = (
  <h2 className="heading heading-medium">
    React Deep Dive
  </h2>
)

An Element has a type, props, and any children nested inside of it. The type can be either a DOM Element (like 'h2', in the above example), or a Component Element (like the Heading described below).

React Component
A React Component is composed of React Elements. It is either a class with a render function, or a stateless functional component. It takes props as an input and returns an element tree as its output. Component names start with a capital letter (Heading vs h2).

const Heading = (props) => {
  return (
   <h2 
    className={`heading heading-${props.size}`}
   >
    {props.children}
   </h2>
  )
}