The definition of “hosting” does not describe just one service, but a number of services which provide different functions to a domain address. Having a site and e-mails, for example, are two separate services even though in the general case they come together, so a lot of people see them as one single service. Actually, each 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, that defines where the site for the domain address is loaded from, while the latter is an alphanumeric string, which shows the server that deals with the emails for the domain. As an illustration, an A record can be 18.104.22.168 and an MX record is mx1.domain.com. Each time you open a site or send an email, the global DNS servers are contacted to check the name servers that a domain has and the traffic/message is first forwarded to that company. When you have custom records on their end, the web browser request or the e-mail will then be directed to the correct server. The reasoning behind using separate records is that the two services use different web protocols and you can have your website hosted by one provider and the emails by another.