The term “hosting” doesn't describe a single service, but a variety of services that provide numerous functions to a domain name. Having a website and emails, as an illustration, are two independent services despite the fact that in the general case they come together, so most people think of them as one single service. The truth is, each domain 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 specifies where the site for the domain address is loaded from, while the second one is an alphanumeric string, which shows the server that manages the emails for the domain address. For example, an A record can be 123.123.123.123 and an MX record is mx1.domain.com. Whenever you open a website or send an email, the global DNS servers are contacted to check the name servers that a domain address has and the traffic/message is first forwarded to that company. If you have custom records on their end, the web browser request or the e-mail will be forwarded to the correct server. The reasoning behind employing separate records is that the two services use different web protocols and you can have your site hosted by one company and the emails by another.