The term “hosting” does not describe one service, but a variety of services which provide a variety of functions to a domain. Having a website and emails, for instance, are two individual services despite the fact that in the general case they come together, so a lot of people see them as one single service. In reality, each and every domain has a several DNS records called A and MX, which show the server that manages each specific service - the first one is a numeric IP address, which specifies where the site for the domain name is loaded from, while the second one is an alphanumeric string, which shows the server that handles the emails for the domain address. For instance, an A record is 123.123.123.123 and an MX record would be mx1.domain.com. Each time you open a site or send an e-mail, 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 browser request or the e-mail will be directed to the correct server. The reasoning behind employing separate records is that the two services work with different web protocols and you could have your website hosted by one provider and the e-mails by another.