With Inertia you build apps just like you've always done with your server-side web framework of choice. You use your framework's existing functionality for routing, controllers, middleware, authentication, authorization, data fetching, and more.
The only thing that's different is your view layer. Instead of using server-side rendering (eg. Blade or ERB templates), the views are JavaScript page components. This allows you to build your entire front-end using React, Vue or Svelte.
But simply creating your front-end in JavaScript doesn't give you a single-page app experience. If you were to click a link, your browser would make a full page visit, which would then cause your client-side framework to reboot on the subsequent page load. This is where Inertia comes in.
At its core Inertia is essentially a client-side routing library. It allows you to make page visits without forcing a full page reload. This is done using the <Link> component, a light wrapper around a normal anchor link. When you click an Inertia link, Inertia intercepts the click and makes the visit via XHR instead. Worth noting, you can also make these visits programmatically in JavaScript using Inertia.visit().
When Inertia makes an XHR visit, the server detects that it's an Inertia visit, and instead of returning a full HTML response, it returns a JSON response with the JavaScript page component name and data (props). Inertia then dynamically swaps out the previous page component with the new page component, and updates the history state.
The end result is a silky smooth single-page experience. 🎉
To learn more about how this is technically done, see the protocol page.