The term “hosting” does not describe one service, but several services that offer various functions to a domain address. Having a website and emails, for instance, are two independent services though in the general case they come together, so many people think of them as one single service. In reality, each and every domain has a several DNS records called A and MX, which show the server that handles each specific service - the former is a numeric IP address, which identifies where the website for the domain name is loaded from, while the second one is an alphanumeric string, which shows the server that deals with the emails for the domain. As an example, an A record is 123.123.123.123 and an MX record can be mx1.domain.com. Each time you open a website 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 browser request or the e-mail will be sent to the correct server. The reasoning behind working with separate records is that the two services employ different web protocols and you may have your website hosted by one provider and the emails by another.