The term “hosting” does not describe a single service, but a set of services which provide various functions to a domain address. Having a site and e-mails, for example, are two separate services although in the general case they come together, so most of the people think of them as one single service. In reality, every domain 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 defines where the site for the domain is loaded from, while the second one is an alphanumeric string, which shows the server that handles the emails for the domain name. For example, an A record would be 220.127.116.11 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 Internet browser request or the e-mail will then be sent to the correct server. The idea behind employing separate records is that the two services use different web protocols and you could have your website hosted by one provider and the e-mails by another.