The definition of “hosting” doesn't describe one service, but a variety of services that provide numerous functions to a domain address. Having a website and emails, 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. The truth is, every single domain name has a number of DNS records called A and MX, which show the server that deals with each particular service - the former is a numeric IP address, that identifies where the site 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 address. For example, an A record would be 123.123.123.123 and an MX record can be mx1.domain.com. Each time 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. In case you have custom records on their end, the web browser request or the email will be forwarded to the correct server. The concept behind using separate records is that the two services employ different web protocols and you could have your site hosted by one service provider and the e-mail messages by another.