Need help to understand how web servers and server side scripts (web applications) interact [on hold]

I am reading about web servers, server side scripting language and web applications, basically trying to get an about the technology involved in web development. I understand that clients (ex:web browsers) communicate with web servers mainly using HTTP protocol.

I am also reading about server side scripting and web applications (I understand interactive web apps are developed using server side languages like PHP, Python, ASP etc). While most of the content that I came across about web servers and web applications mentioned that when we enter a URL ex- http://ift.tt/1ezuwGJ in the browser, a request is sent to the server "stackoverflow" and it responds with the page present in the location questions/ask.

My confusions arises here - stackoverflow (equivalent IP xx.xx.xx.xx) is a server - and stackoverflow is also a web application developed using one of the server side scripting languages or web app development languages.

How do we distinguish between server and web application here ?

1. My question is how does the server "Stackoverflow" communicate with the web application called "stackoverflow" to generate dynamic content specific to a user ?

2. All I know is server responds to users requests. How does the server respond with information specific to a user ? There must be some way that the server is taking to stackoverflow web application ? How does this talking happen ?

3. When stackoverflow web application is hosted on a server, how does the server run this application ? Is it like everytime user requests something, a program is executed in the server machine ? The server takes the output of this program and returns in the HTTP response to the client ?

4. What languages are used to build servers ? This question is because, I read somewhere that servers are usually built using C,C++ or Java. Then how can we host a PHP web application on a server written using C,C++ or Java ? I cannot piece together how a server talks to a web application.

I know most of these questions sound trivial, might even scream of lack of basic understanding. But everyone has to start somewhere and I have the willingness to do whatever is needed to learn.

I look forward to some help in clear understanding of these concepts. You may also point to some web resources from where I can read/watch videos and gain insight into these technologies.

Thank you very much in advance.