different between router in node js

Question

what is difference between routing method in express() and in router() function in express()?

const express=require('express');
const app=express();
const router=express.router();


//routing with express
app.get(/,(req,res)=>{
res.send('Get method with express');
});


//routing with router function in express
router.get('/',(req,res)=>{
res.send('Get method with router in express')
});

which on for routing is better? which one should i use?