The term “hosting” doesn't describe a single service, but several services which provide different functions to a domain address. Having a website and emails, as an example, are two individual services although in the general case they come together, so a lot of people consider them as one single service. In fact, every domain has a couple of DNS records called A and MX, which show the server that manages each particular 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. For example, an A record can be 184.108.40.206 and an MX record would be 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 address has and the traffic/message is first forwarded to that company. If you have custom records on their end, the browser request or the email will then be directed to the correct server. The concept behind working with separate records is that the two services use different web protocols and you can have your website hosted by one provider and the emails by another.