A Stable, Free And Open-Source HTTP Server
nginx is an HTTP, reverse proxy, mail proxy, and generic TCP/UDP proxy server. Practically speaking, that means it's a rock-solid base for a number of different web sites. For a long time, nginx has seen use on sites with heavy loads, from Yandex and Mail.ru in its infancy to Netflix today.
Heavy On Security, Light On Features
Security is a notable concern for nginx's developers. Not because the software isn't secure: quite the opposite. Patches are signed using keys to verify that no one is tampering with nginx itself, and security issues are reported and fixed quite quickly. On the other hand, while you won't take on many risks by using the software, you will probably have some learning to do. It's reasonably comfortable to use this software if you've used other server software before, but otherwise, you'll want to read the FAQ and documentation thoroughly, and contact the developers for support when you truly need it.
A Rock Solid Choice
Overall, nginx won't be winning any awards for ease of use or number of features. It can serve a lot of different purposes, from serving up HTTP web pages to acting as a private proxy server. Within each role it doesn't do anything fancy, but it does its job quite well: it boasts high uptime and increasingly good security.