Differences and Similarities Between Lumen and Laravel

Update (5.2)

With the latest version of Lumen (5.2) the Microframework focuses on Stateless APIs.
The documentation states:

Lumen 5.2 represents a shift on slimming Lumen to focus solely on serving stateless, JSON APIs. As such, sessions and views are no longer included with the framework. If you need access to these features, you should use the full Laravel framework.

Original answer (<= 5.1)

Lumen is all about speed. It's faster and can handle more requests per second than Laravel.

Laravel is a framework that glues together a lot of components (3rd party and Laravels own components). Now Lumen makes use of a lot of the same components, but strips down the bootstrapping process to a minimum. You could say it is a "different glue", the components (and therefore a lot of the features) are mostly the same.

The performance improvement is achieved by taking away some of the flexibility of the framework in terms of configuration and altering the default boot process.

Besides that, more features are disabled by default and have to be activated before they can be used. As an example: Facades (like DB::table())
You first need to uncomment this line in bootstrap/app.php to enable them:

// $app->withFacades();

The same goes for Dotenv environment files and Eloquent.

For routing Lumen uses nikic/FastRoute instead of symfonys router because it performs a lot better and gives the micro-framework another big boost.

Other than that pretty much everything is the same as in Laravel.

Good reads on Lumen

• Documentation (especially the introduction)
• Article from Matt Stauffer
• Interview with Taylor Otwell (especially the last question)
• Reddit discussion
• Laracast: Introducing Lumen

Lumen is not designed to replace Laravel, rather, it is a more specialized (and stripped-down) framework designed for micro-services and APIs. It took away unneeded features for an API such as HTTP sessions and cookies, and also limited the number of configuration options. Out-of-the-box, Lumen sacrified the flexibility of Laravel for speed.

However, you can add Laravel components to Lumen to extend it, so it can be used for more than just micro-services and API. However, if your goal is to extend Lumen to become a website, you might as well use Laravel instead.

They also have different use cases. Lumen and Laravel are meant to work together. For APIs and services are frequently get called, use Lumen. For user-facing applications, use Laravel.

This answer is taken from a blog post I wrote that explains the difference between Lumen and Laravel.

Quote from Matt Stauffer

Lumen has the same foundation as Laravel, and many of the same components. But Lumen is built for microservices, not so much for user-facing applications (although it can be used for anything.) As such, frontend niceties like Bootstrap and Elixir and the authentication bootstrap and sessions don't come enabled out of the box, and there's less flexibility for extending and changing the bootstrap files.

You can read more here