The word “hosting” does not describe a single service, but a variety of services that provide various functions to a domain. Having a site and emails, as an example, are two individual services though in the general case they come together, so a lot of people think of them as one single service. Actually, each domain has a couple of DNS records called A and MX, which show the server that handles each specific service - the first one is a numeric IP address, that identifies 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 name. As an illustration, an A record would be 123.123.123.123 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 Internet domain has and the traffic/message is first directed to that company. When you have custom records on their end, the web browser request or the email will then be forwarded to the correct server. The reasoning behind working with separate records is that the two services employ different web protocols and you could have your website hosted by one provider and the emails by another.