The definition of “hosting” does not describe a single service, but a variety of services which offer numerous functions to a domain name. Having a site and e-mails, as an example, are two individual services although in the general case they come together, so most of the people see them as one single service. In reality, each domain has a several DNS records called A and MX, which show the server that handles each particular service - the first one is a numeric IP address, that defines where the website for the domain name is loaded from, while the second one is an alphanumeric string, which shows the server that manages the emails for the domain address. As an example, an A record can be 220.127.116.11 and an MX record is mx1.domain.com. Every time you open a site or send an e-mail, 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. If you have custom records on their end, the web browser request or the email will then be forwarded to the correct server. The concept behind using separate records is that the two services use different web protocols and you may have your site hosted by one provider and the e-mails by another.