The term “hosting” does not describe one service, but a variety of services which provide different functions to a domain address. Having a site and e-mails, as an illustration, are two independent services although in the general case they come together, so a lot of people see them as one single service. In fact, each and every domain name 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 identifies where the site for the domain is loaded from, while the latter is an alphanumeric string, which shows the server that manages the emails for the domain name. For instance, an A record is 188.8.131.52 and an MX record is mx1.domain.com. Every time you open a site 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 e-mail will be forwarded to the correct server. The idea behind using separate records is that the two services employ different web protocols and you could have your site hosted by one provider and the e-mail messages by another.