The definition of “hosting” does not describe one service, but a set of services which offer numerous functions to a domain address. Having a site and emails, as an illustration, are two individual services although in the general case they come together, so many people think of them as one single service. Actually, every domain name has a number of DNS records called A and MX, which show the server that handles each particular service - the former is a numeric IP address, that specifies where the site for the domain name is loaded from, while the latter is an alphanumeric string, which shows the server that manages the emails for the domain. For example, an A record can be 18.104.22.168 and an MX record would 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 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 forwarded to the correct server. The concept behind employing separate records is that the two services use different web protocols and you may have your website hosted by one provider and the emails by another.