Struggling with the concepts of callbacks used together with apis (application programming interfaces)l

app.js

/*  
Using accessing an external local file ie one on this computer
*/
var weather = require('./weather.js');
var location = require('./location2.js');
/*
weather(function(currentWeather) {
	currentWeather;
});

location(function(location) {
	// We want the following properties
	// (1) city name (2) longitude + lattitude
	console.log('city' + location.city);
	console.log('long/lat' + location.loc);
});
*/

location(function (location){
	if (!location){
		console.log('Location is unknown!');
	} else {
		console.log('City name:' + location.city);
		console.log('Long/lat: ' + location.loc);	
	}
});

location2.js

/* 
This file runs in association with app.js
*/

	module.exports = function(callback){
		var request = require('request');
		var url = 'http://ipinfo.io';

		/*
				The request modules takes 2 arguments
				The request module takes an object as an argument.
				The object has two fields, a url key and a json key
				*/

				/*
				body = visible data on the web page.
				4 is the number of spaces to indent json data by
		*/
		request({url: url, json: true}, function(error, response, body){
				if(error){
					callback ();
				} else {
					// This is how you pass the 'body' argument
					callback(body);
				}
			});	
	};

For file 'location2.js' is 'callback' mentioned on line 5 the same as callback referenced on lines 21 and 24? Why is callback on line 5 written as 'callback' without any brackets is ()?

Why is there no statement like function callback() {...} ?