Domain Name System - translates human-readable website names into computer addresses. Like a phone book that converts 'Pizza Place' into the actual phone number.
When you type 'google.com', DNS converts it to an IP address like 142.250.185.78 so your browser knows which server to connect to.
All four are managed authoritative DNS services used to host public/private DNS zones, create records (A/AAAA/CNAME/MX/TXT), and route traffic to applications. AWS Route 53 also includes built-in health checks and multiple routing policies; other providers offer similar capabilities via their DNS plus monitoring/traffic services.