The word “hosting” does not describe a particular service, but a variety of services which provide a variety of functions to a domain. Having a website and e-mails, as an example, are two individual services even though in the general case they come together, so most of the people consider them as one single service. In fact, each and every domain name 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, which specifies where the site for the domain address is loaded from, while the second one is an alphanumeric string, which shows the server that handles the emails for the domain address. As an example, an A record is 220.127.116.11 and an MX record is mx1.domain.com. Every time you open a website or send an e-mail, the global DNS servers are contacted to check the name servers that a domain has and the traffic/message is first forwarded to that company. If you have custom records on their end, the browser request or the e-mail will then be forwarded to the correct server. The idea behind employing separate records is that the two services employ different web protocols and you could have your website hosted by one service provider and the emails by another.