express() vs express.router()

var express = require('express'),
    dogs    = require('./routes/dogs'),
    cats    = require('./routes/cats'),
    birds   = require('./routes/birds');

var app = express();

app.use('/dogs',  dogs);
app.use('/cats',  cats);
app.use('/birds', birds);

app.listen(3000);

var express = require('express');
var fs  = require('fs');
var app= express();
 app.get('/index/profile/:id',function(req,res){
    //  res.send('profile with id' + req.params.id)

    
    
 });
app.listen(3000,'127.0.0.1');
console.log('lsitng');

var express = require('express');

var router = express.Router();

router.get('/', function(req, res) {
    res.send('GET handler for /dogs route.');
});

router.post('/', function(req, res) {
    res.send('POST handler for /dogs route.');
});

module.exports = router;