The definition of “hosting” doesn't describe a particular service, but several services which offer different functions to a domain name. Having a website and e-mails, for example, are two independent services despite the fact that in the general case they come together, so most of the people think of them as one single service. The truth is, each and every domain has a couple of DNS records called A and MX, which show the server that deals with each specific service - the former is a numeric IP address, which identifies where the site for the domain is loaded from, while the second one is an alphanumeric string, which shows the server that handles the emails for the domain name. As an example, an A record would be 18.104.22.168 and an MX record can be mx1.domain.com. Each time you open a website or send an e-mail, the global DNS servers are contacted to check the name servers that a domain address has and the traffic/message is first forwarded to that company. When you have custom records on their end, the browser request or the e-mail will then be directed to the correct server. The idea behind working with separate records is that the two services employ different web protocols and you can have your site hosted by one provider and the e-mail messages by another.