Let's see if I can't help a little:
Maps your domain name to the system/systems that provide services.
"renting" servers that provide the services that your registration points to.
The idea is that you register a name and that references a DNS server. Surprisingly that usually points to a DNS server at your HOSTING company.
Now, hosting is where you define where things go. www.mysite.com goes to a web server, blog.mysite.com goes to a wordpress site, mail.mysite.com goes to the mail server. All of these services are provided by your hosting company.
I hope this helped a little, it really is confusing. It is why it's convenient to register and host at the same place, but there are reasons why you might not want to do that too.
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