Kind of like asking what's the difference between using a map vs using a car.
Do some basic reading (even Wikipedia). Basically when you type leftypol.org, your computer needs to ask a DNS for the IP address where you can access that site. It's like a phone book for websites. An issue is that using the Google DNS or your ISPs default DNS let's them know which domains you visited and when.
A VPN is a protective layer/tunnel, businesses often use then to secure remote connections (so a worker using cafe WiFi can't easily be tapped by a random WiFi user), home users often use them to hide info from their ISP, have an IP in a different place or country to their own (e.g. geoblocks on Netflix) or otherwise helping stop online stalking.