The word “hosting” doesn't describe just one service, but a set of services that provide numerous functions to a domain. Having a site and e-mails, for example, are two independent services even though in the general case they come together, so a lot of people see them as one single service. In fact, every domain name has a number of DNS records called A and MX, which show the server that manages each particular service - the former is a numeric IP address, that specifies where the website for the domain name is loaded from, while the latter is an alphanumeric string, which shows the server that manages the emails for the domain. For instance, an A record is 18.104.22.168 and an MX record can 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 Internet domain has and the traffic/message is first directed to that company. If you have custom records on their end, the web browser request or the email will then be sent to the correct server. The idea behind using separate records is that the two services work with different web protocols and you can have your site hosted by one provider and the e-mail messages by another.