The term “hosting” doesn't describe a particular service, but a set of services that offer a variety of functions to a domain name. Having a site and emails, for instance, 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 has a several DNS records called A and MX, which show the server that deals with each particular service - the first one is a numeric IP address, which defines where the website 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 name. As an illustration, an A record can be 220.127.116.11 and an MX record would be 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 domain name has and the traffic/message is first forwarded to that company. When you have custom records on their end, the 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 use different web protocols and you can have your website hosted by one service provider and the e-mails by another.