The term “hosting” doesn't describe one service, but a set of services that provide numerous functions to a domain name. Having a site and e-mails, as an illustration, are two individual services though in the general case they come together, so most of the people see them as one single service. Actually, each domain name has a several DNS records called A and MX, which show the server that handles each specific service - the former is a numeric IP address, which identifies where the site for the domain is loaded from, while the latter is an alphanumeric string, which shows the server that manages the e-mails for the domain. As an example, an A record would be 123.123.123.123 and an MX record can be mx1.domain.com. Each time you open a website 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 forwarded to that company. In case you have custom records on their end, the Internet browser request or the e-mail will then be sent to the correct server. The reasoning behind employing separate records is that the two services employ different web protocols and you could have your site hosted by one company and the e-mails by another.