Preface

This answer uses the dynamic routing approach embraced in react-router v4+. Other answers may reference the previously-used "static routing" approach that has been abandoned by react-router.

Solution

react-router is a great solution. You create your pages as Components and the router swaps out the pages according to the current URL. In other words, it replaces your original page with your new page dynamically instead of asking the server for a new page.

For web apps I recommend you read these two things first:

• Full Tutorial
• The react-router docs; it will help you get a better understanding of how Router works.

Summary of the general approach:

1 - Add react-router-dom to your project:

Yarn

yarn add react-router-dom

or NPM

npm install react-router-dom

2 - Update your index.js file to something like:

import { BrowserRouter } from 'react-router-dom';

ReactDOM.render((
  <BrowserRouter>
    <App /> {/* The various pages will be displayed by the `Main` component. */}
  </BrowserRouter>
  ), document.getElementById('root')
);

3 - Create a Main component that will show your pages according to the current URL:

import React from 'react';
import { Switch, Route } from 'react-router-dom';

import Home from '../pages/Home';
import Signup from '../pages/Signup';

const Main = () => {
  return (
    <Switch> {/* The Switch decides which component to show based on the current URL.*/}
      <Route exact path='/' component={Home}></Route>
      <Route exact path='/signup' component={Signup}></Route>
    </Switch>
  );
}

export default Main;

4 - Add the Main component inside of the App.js file:

function App() {
  return (
    <div className="App">
      <Navbar />
      <Main />
    </div>
  );
}

5 - Add Links to your pages.

(You must use Link from react-router-dom instead of just a plain old <a> in order for the router to work properly.)

import { Link } from "react-router-dom";
...
<Link to="/signup">
  <button variant="outlined">
    Sign up
  </button>
</Link>

(Make sure to install react-router using npm!)

To use react-router, you do the following:

1. Create a file with routes defined using Route, IndexRoute components

2. Inject the Router (with 'r'!) component as the top-level component for your app, passing the routes defined in the routes file and a type of history (hashHistory, browserHistory)

3. Add {this.props.children} to make sure new pages will be rendered there
4. Use the Link component to change pages

Step 1 routes.js

import React from 'react';
import { Route, IndexRoute } from 'react-router';

/**
 * Import all page components here
 */
import App from './components/App';
import MainPage from './components/MainPage';
import SomePage from './components/SomePage';
import SomeOtherPage from './components/SomeOtherPage';

/**
 * All routes go here.
 * Don't forget to import the components above after adding new route.
 */
export default (
  <Route path="/" component={App}>
    <IndexRoute component={MainPage} />
    <Route path="/some/where" component={SomePage} />
    <Route path="/some/otherpage" component={SomeOtherPage} />
  </Route>
);

Step 2 entry point (where you do your DOM injection)

// You can choose your kind of history here (e.g. browserHistory)
import { Router, hashHistory as history } from 'react-router';
// Your routes.js file
import routes from './routes';

ReactDOM.render(
  <Router routes={routes} history={history} />,
  document.getElementById('your-app')
);

Step 3 The App component (props.children)

In the render for your App component, add {this.props.children}:

render() {
  return (
    <div>
      <header>
        This is my website!
      </header>

      <main>
        {this.props.children}
      </main>

      <footer>
        Your copyright message
      </footer>
    </div>
  );
}

Step 4 Use Link for navigation

Anywhere in your component render function's return JSX value, use the Link component:

import { Link } from 'react-router';
(...)
<Link to="/some/where">Click me</Link>

This is a broad question and there are multiple ways you can achieve this. In my experience, I've seen a lot of single page applications having an entry point file such as index.js. This file would be responsible for 'bootstrapping' the application and will be your entry point for webpack.

index.js

import React from 'react';
import ReactDOM from 'react-dom';
import Application from './components/Application';

const root = document.getElementById('someElementIdHere');

ReactDOM.render(
  <Application />,
  root,
);

Your <Application /> component would contain the next pieces of your app. You've stated you want different pages and that leads me to believe you're using some sort of routing. That could be included into this component along with any libraries that need to be invoked on application start. react-router, redux, redux-saga, react-devtools come to mind. This way, you'll only need to add a single entry point into your webpack configuration and everything will trickle down in a sense.

When you've setup a router, you'll have options to set a component to a specific matched route. If you had a URL of /about, you should create the route in whatever routing package you're using and create a component of About.js with whatever information you need.