The word “hosting” doesn't describe only one service, but several services which provide various functions to a domain name. Having a website and emails, as an example, are two separate services despite the fact that in the general case they come together, so most people think of them as one single service. In reality, each and 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, that specifies where the site for the domain is loaded from, while the latter is an alphanumeric string, which shows the server that manages the emails for the domain. As an example, an A record would be 220.127.116.11 and an MX record can be 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 Internet domain has and the traffic/message is first forwarded to that company. When you have custom records on their end, the Internet browser request or the e-mail will then be forwarded to the correct server. The reasoning behind using separate records is that the two services use different web protocols and you can have your site hosted by one company and the e-mails by another.