The term “hosting” does not describe a single service, but several services that offer different functions to a domain. Having a site and emails, as an example, are two individual services although in the general case they come together, so many people think of 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 specific service - the first one is a numeric IP address, that defines where the website for the domain address 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 would be 220.127.116.11 and an MX record can be mx1.domain.com. Whenever 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 directed to that company. In case you have custom records on their end, the web browser request or the email will then be forwarded to the correct server. The idea behind working with separate records is that the two services employ different web protocols and you could have your website hosted by one provider and the e-mails by another.