Why do I need Nginx and something like Gunicorn?

Overly simplified: You need something that executes Python but Python isn't the best at handling all types of requests.

[disclaimer: I'm a Gunicorn developer]

Less simplified: Regardless of what app server you use (Gunicorn, mod_wsgi, mod_uwsgi, cherrypy) any sort of non-trivial deployment will have something upstream that will handle the requests that your Django app should not be handling. Trivial examples of such requests are serving static assets (images/css/js).

This results in two first tiers of the classic "three tier architecture". Ie, the webserver (Nginx in your case) will handle many requests for images and static resources. Requests that need to be dynamically generated will then be passed on to the application server (Gunicorn in your example). (As an aside, the third of the three tiers is the database)

Historically speaking, each of these tiers would be hosted on separate machines (and there would most likely be multiple machines in the first two tiers, ie: 5 web servers dispatch requests to two app servers which in turn query a single database).

In the modern era we now have applications of all shapes and sizes. Not every weekend project or small business site actually needs the horsepower of multiple machines and will run quite happily on a single box. This has spawned new entries into the array of hosting solutions. Some solutions will marry the app server to the web server (Apache httpd + mod_wsgi, Nginx + mod_uwsgi, etc). And its not at all uncommon to host the database on the same machine as one of these web/app server combinations.

Now in the case of Gunicorn, we made a specific decision (copying from Ruby's Unicorn) to keep things separate from Nginx while relying on Nginx's proxying behavior. Specifically, if we can assume that Gunicorn will never read connections directly from the internet, then we don't have to worry about clients that are slow. This means that the processing model for Gunicorn is embarrassingly simple.

The separation also allows Gunicorn to be written in pure Python which minimizes the cost of development while not significantly impacting performance. It also allows users the ability to use other proxies (assuming they buffer correctly).

As to your second question about what actually handles the HTTP request, the simple answer is Gunicorn. The complete answer is both Nginx and Gunicorn handle the request. Basically, Nginx will receive the request and if it's a dynamic request (generally based on URL patterns) then it will give that request to Gunicorn, which will process it, and then return a response to Nginx which then forwards the response back to the original client.

So in closing, yes. You need both Nginx and Gunicorn (or something similar) for a proper Django deployment. If you're specifically looking to host Django with Nginx, then I would investigate Gunicorn, mod_uwsgi, and maybe CherryPy as candidates for the Django side of things.

I liked this explanation in its simplicity:

Nginx will face the outside world. It will serve media files (images, CSS, etc) directly from the file system. However, it can't talk directly to Django applications; it needs something that will run the application, feed it requests from the web, and return responses.

That's Gunicorn's job. Gunicorn will create a Unix socket, and serve responses to nginx via the wsgi protocol - the socket passes data in both directions:

The outside world <-> Nginx <-> The socket <-> Gunicorn

https://gist.github.com/Atem18/4696071

I'm looking for an overly simplified answer...

Do I need both Nginx and something like Gunicorn to deploy Django apps on Nginx?

If so, what actually handles the HTTP requests?

Overly simplified answer:

YES.

Both Nginx and Gunicorn.

Since you are deploying on Nginx, of course you need Nginx.

Since you are deploying Django, which is a web framework, you need something bridging the talk between the web server (Nginx) and the web framework (Django). In Python world, such a thing is called a WSGI server (but think it like a middle ware), examples of which include Gunicorn and uWSGI. When handling a request, Nginx proxies the request to Gunicorn or uWSGI, which in turn calls Django code and returns the response.

This document and Paul's answer will help you learn it better.

Gunicorn is a waste of resources. You can simply proxy pass to django listening on a port instead of running gunicorn on top django and again nginx on top of all that. In benchmarks, I have seen very noticeable speed increase. Nginx can easily handle direct requests to django. Gunicorn is nothing else than a flyover(actually a slower flyover) above the normal road. It just sits and eats your resources and tries to claim to be powering your website.

nginx basically buffers all the requests and handles the static file requests by itself(if you have configured it like that). And proxies all the dynamic content to another server.(gunicorn/django).

Gunicorn has no other use than to just pass the request to application. It's like a straw you can drink from glass directly or drink from the straw at a limited pace(here the person drinking is django). And nginx is the waiter who brought you the glass of juice.

I have benchmarked and found this - with gunicorn: 22k req / s without gunicorn: 34k req / s

Your site will need the extra req/s in heavy load.