router vs app express

1var express = require('express'),
2    dogs    = require('./routes/dogs'),
3    cats    = require('./routes/cats'),
4    birds   = require('./routes/birds');
5
6var app = express();
7
8app.use('/dogs',  dogs);
9app.use('/cats',  cats);
10app.use('/birds', birds);
11
12app.listen(3000);
13