It’s a big difference!

1.1.1.1 is a DNS service wrapped in a VPN. In other words, your DNS queries (so when you type http://www.quora.com into the browser, and the DNS server sends back the IP address that your browser then goes to) go through a VPN tunnel, between your computer/mobile-device to 1.1.1.1’s data-centre, such that no one between you and 1.1.1.1 - like your ISP - can see your DNS queries.

To be clear, it’s ONLY your DNS queries that are enveloped in that VPN tunnel; ALL your other traffic - the actual websites, emails, music/movies/tv/etc - gets out onto the internet from your computer, or comes back from the immediately into your computer, at the ‘gateway’ your ISP controls.

The reasons people might have for using 1.1.1.1 is that, by default, you normally end up using your ISP’s DNS servers, but many ISPs log all their users’ DNS queries (often to sell website traffic metrics to data harvesting companies, marketers, etc), and some even filter their DNS results, for a range of reasons, be they commercial or censorship or whatever.

By having your DNS queries encrypted as they leave your computer/device and go to somewhere that’s ostensibly more trustworthy than your ISP, you avoid that logging, filtering, censorship, whatever the case may be.

Also, 1.1.1.1’s DNS servers are pretty fast, whereas YMMV when it comes to the responsiveness of your ISP’s DNS servers.