Name servers & DNS
DNS, which stands for Domain Name System, can be confusing. It's made up of many different elements which control different aspects of your domain name. In a nutshell, when a domain is registered, basic details are published to a WHOIS database and those details include the Name Servers.
When someone types in your domain name into a browser, the browser looks up the information it needs to reach the website. It first checks WHOIS to see which name servers to look up. Then, it looks up Name Server Records within and loads the website.
Changes to DNS take time to take effect. This is referred to as DNS propagation and it applies to both Name Server changes as well as changes to Name Server Records. Some changes take effect quickly, like adding a new record, while other changes can take up to 48 hours or more to propagate globally.
DNS Basics
Common Setup Guides for Services
Modifying Name Servers and Records