NGINX is an extremely fast webserver. It is used to serve up all your static files as quickly as possible. This includes your app's compiled images, javascript, stylesheets, and other assets.
Passenger is used to serve your application through NGINX. It's wonderfully easy to setup and configure and can run more than just Ruby apps. You can even manually deploy Python and NodeJS apps to your server as well if you like. We will be adding official support for Python and Node apps in the near future.