Most of the getting started examples of ExpressJs include this piece of code
var express = require('express'); var app = express(); ... app.listen(1337);
app.listen is just a shortcut for:
var express = require('express'); var app = express(); var http = require('http'); http.createServer(app).listen(1337);
http.createServer accept a function which is known as the handler. The handler takes 2 parameters request and response as inputs, then manipulating them inside it's scope to do various things.
app = express() is a function, taking place as the handler and dealing with request, response through a set of special components referred as middlewares.