The term “hosting” does not describe one service, but a set of services which provide various functions to a domain name. Having a site and e-mails, as an example, are two individual services though in the general case they come together, so many people think of them as one single service. In reality, every single domain name has a several DNS records called A and MX, which show the server that deals with each specific service - the former is a numeric IP address, which defines where the website for the domain name is loaded from, while the latter is an alphanumeric string, which shows the server that deals with the emails for the domain name. For example, an A record can be 188.8.131.52 and an MX record can 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 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 email will be directed 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 e-mail messages by another.