Phusion Passenger extends Nginx and turns it into an application server. An application server is a program that runs application code, so in this case it allows Nginx to run run Ruby/Python code. Likewise, the Apache version of Phusion Passenger turns Apache into an app server that can run Ruby/Python code.

The benefits are not lost. Phusion Passenger integrates into Nginx to give you the benefits of both. For example one of the tasks of Nginx is to buffer HTTP requests and responses in order to protect apps from slow HTTP connections. Phusion Passenger fully makes use of this Nginx feature and even extends it.

> if I install Passanger to be able to run Rails apps on on Nginx, are these benefits lost?

No. but you will see the benefits only at a high req/min, because with Apache it would have to spawn new threads to deal with so many requests, taking up memory and other resources, while nginx just sends the requests directly to the passenger instances as needed. basically nginx would use less memory.

> What exact is Passanger? passenger is a module that understands and handles rails requests. So when a request comes in to nginx for an image, css or other static asset, nginx will find it and send it back, when the request is for a rails controller/action, then it will send it to a passenger instance to process and then return the results to you

hope that helps