The term “hosting” does not describe one service, but a set of services that offer numerous functions to a domain name. Having a site and e-mails, for example, are two individual services although in the general case they come together, so a lot of people see them as one single service. Actually, every domain has a couple of DNS records called A and MX, which show the server that manages each particular service - the first one is a numeric IP address, which defines where the site for the domain name is loaded from, while the latter is an alphanumeric string, which shows the server that manages the e-mails for the domain name. As an example, an A record is 220.127.116.11 and an MX record can be mx1.domain.com. Whenever you open a website or send an email, the global DNS servers are contacted to check the name servers that a domain name has and the traffic/message is first forwarded to that company. In case you have custom records on their end, the web browser request or the email will then be forwarded to the correct server. The idea behind using separate records is that the two services use different web protocols and you can have your site hosted by one service provider and the e-mail messages by another.