The definition of “hosting” doesn't describe only one service, but a variety of services which offer different functions to a domain. Having a site and e-mails, as an illustration, are two separate services although in the general case they come together, so a lot of people see them as one single service. In fact, each and every domain name has a couple of DNS records called A and MX, which show the server that manages each specific service - the former is a numeric IP address, that specifies where the site for the domain is loaded from, while the second one is an alphanumeric string, which shows the server that handles 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. Each time you open a website or send an e-mail, 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. In case you have custom records on their end, the browser request or the email will be forwarded to the correct server. The concept behind working with separate records is that the two services use different web protocols and you can have your site hosted by one provider and the e-mail messages by another.