The definition of “hosting” does not describe a particular service, but a number of services which offer different functions to a domain address. Having a site and emails, for instance, are two individual services despite the fact that in the general case they come together, so most people see them as one single service. In fact, every single domain has a couple of DNS records called A and MX, which show the server that handles each specific service - the former is a numeric IP address, that specifies where the website for the domain address is loaded from, while the latter is an alphanumeric string, which shows the server that manages the e-mails for the domain name. For instance, an A record is 126.96.36.199 and an MX record is mx1.domain.com. Whenever you open a site or send an e-mail, 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 be sent to the correct server. The idea behind employing separate records is that the two services employ different web protocols and you can have your site hosted by one provider and the e-mail messages by another.