The term “hosting” does not describe one service, but several services that offer different functions to a domain. Having a website and emails, for instance, are two independent services despite the fact that in the general case they come together, so most of the people think of them as one single service. Actually, each and every domain name has a several DNS records called A and MX, which show the server that manages each particular service - the first one is a numeric IP address, which identifies where the website for the domain address is loaded from, while the second one is an alphanumeric string, which shows the server that deals with the emails for the domain. For instance, an A record can be 123.123.123.123 and an MX record is mx1.domain.com. Each time you open a site 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. When you have custom records on their end, the web browser request or the e-mail will then be sent to the correct server. The reasoning behind using separate records is that the two services work with different web protocols and you can have your site hosted by one service provider and the e-mail messages by another.