The term “hosting” does not describe one service, but several services that provide numerous functions to a domain address. Having a website and emails, as an example, are two separate services though in the general case they come together, so most people consider them as one single service. The truth is, each domain name has a couple of DNS records called A and MX, which show the server that manages each specific service - the first one is a numeric IP address, that defines where the site for the domain is loaded from, while the second one is an alphanumeric string, which shows the server that manages the e-mails for the domain address. As an example, an A record is 220.127.116.11 and an MX record would be mx1.domain.com. Every time you open a site 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. When you have custom records on their end, the browser request or the e-mail will then be directed to the correct server. The idea behind employing separate records is that the two services use different web protocols and you can have your website hosted by one provider and the e-mail messages by another.