The term “hosting” doesn't describe only one service, but a number of services that offer numerous functions to a domain name. Having a website and e-mails, for example, are two independent services even though in the general case they come together, so most people think of them as one single service. The truth is, every single domain name has a number of DNS records called A and MX, which show the server that manages each particular service - the first one is a numeric IP address, which specifies where the website for the domain is loaded from, while the second one is an alphanumeric string, which shows the server that deals with the e-mails for the domain. As an example, an A record is 18.104.22.168 and an MX record is mx1.domain.com. Whenever you open a site or send an email, the global DNS servers are contacted to check the name servers that a domain has and the traffic/message is first forwarded to that company. In case you have custom records on their end, the web browser request or the email will then be sent to the correct server. The concept behind employing separate records is that the two services employ different web protocols and you could have your site hosted by one service provider and the e-mails by another.