If you’re looking to have a live website or email you’re going to need two main components if the title didn’t give it away those would be your domain registration and the Web Hosting.
These perform separate tasks but unify to provide the finished result (a functioning website and or email) the domain registration itself is purely to provide you with exclusive use to a domain name that can be used to route information and traffic to your desired hosting. The hosting then takes over as the physical hardware and software that stores manage and runs your website and or email.
You can run multiple sites off of one hosting account, and you can own multiple domains that point to one site, as such these are billed separately to ensure you can create a system that caters to your needs.
If you are using the popular website builders Squarespace or Wix, your web hosting is already provided by them as part of the site builder service.