The word “hosting” does not describe a particular service, but a number of services which provide different functions to a domain. Having a site and e-mails, as an illustration, are two separate services although in the general case they come together, so many people think of them as one single service. Actually, every domain name 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, that identifies where the site for the domain name is loaded from, while the latter is an alphanumeric string, which shows the server that deals with the e-mails for the domain address. As an illustration, an A record would 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 address 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 e-mail will be forwarded to the correct server. The idea behind employing separate records is that the two services use different web protocols and you can have your website hosted by one company and the emails by another.