What is Full Stack Javascript?

Of the languages you mentioned, I have the most experience with PHP on Apache. Let me try and give you an answer in relation to that. With PHP on Apache, your browser makes an HTTP request to an Apache server. Let's say, http://example.com/index.php. The Apache server at example.com looks at the request, locates the resource, in this case index.php, parses the file, interprets any <?php ?> scripts it finds, and sends the result back to your browser. With server side javascript, you use javascript to create the server. Your browser makes a request to the javascript server, the server processes the request using the javascript code you programmed it with, and the results are sent back to your browser.

For example, here is a quick and dirty http nodejs server:

var http = require('http') 
    , html = '<h1>Hello World</h1>'; 

http.createServer(function (req, res) {
  res.writeHead(200, {'Content-Type': 'text/html'});
  res.end(html);
}).listen(3000);

If this were saved in a file called app.js, you would run this using node like so: node ./app.js. This server will listen on TCP port 3000, not the typical port 80, and doesn't really serve a valid HTML page, but hopefully you get the idea. Something similar in PHP, once you set up your Apache server, would look something like this:

<?php echo "<h1>Hello World</h1>"; ?>

Hopefully that gives you some idea as to the differences. There are other differences in how the servers themselves operate. For instance, Apache usually uses separate process to serve multiple requests. Each process will block incoming connections until it is finishes serving the request it is currently working on. Node uses a non-blocking event loop. Instead of blocking requests until it is finished serving the current request, the event loop sends the request to your code with a callback function to execute when any processing is done. Each has their strengths and weaknesses, and in the end you should probably learn a few different languages and the best use cases for each. Keep your toolbox full. If all you have is a hammer, everything looks like a nail.

Hope that makes sense.