The term “hosting” does not describe a particular service, but a number of services that offer various functions to a domain. Having a site and e-mails, for example, are two individual services although in the general case they come together, so most people consider them as one single service. Actually, every domain name has a several DNS records called A and MX, which show the server that manages each specific service - the former is a numeric IP address, which defines where the website for the domain address is loaded from, while the latter is an alphanumeric string, which shows the server that manages the emails for the domain name. For instance, an A record can be 123.123.123.123 and an MX record is mx1.domain.com. Whenever you open a site 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 directed to that company. In case you have custom records on their end, the Internet browser request or the e-mail will then be directed to the correct server. The idea behind using separate records is that the two services employ different web protocols and you may have your site hosted by one provider and the e-mails by another.