What is the difference between "app.get/post/put/delete()" and "router.get/post/put/delete()"?

I am a new NodeJS learner. I have seen in some codes they have used:

const express = require('express');
const app = express();
app.get/post/put/delete(....

And in some other codes, they've used:

const express = require('express');
const router = express.Router();
router.get/post/put/delete(...

What is the difference between these two methods and which one is better to use?