var express = require('express');
var app = express();
Is it a method or a constructor?
Neither; it's a function, although if you said "method" I don't think anyone would give you a hard time.
new operator. Even though other functions may create things, we don't typically call them "constructors" to avoid confusion. Sometimes they may be "creator" or "builder" functions.
Where does it come from?
ExpressJS is a NodeJS module;
express is the name of the module, and also the name we typically give to the variable we use to refer to its main function in code such as what you quoted. NodeJS provides the
require function, whose job is to load modules and give you access to their exports. (You don't have to call the variable
express, you can do
var foo = require('express'); and use
foo instead, but convention is that you'd use the module's name, or if only using one part of a module, to use the name of that part as defined by the module's documentation.)