The term “hosting” doesn't describe only one service, but a variety of services that offer numerous functions to a domain. Having a site and emails, for example, are two independent services although in the general case they come together, so many people consider 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 deals with each specific service - the former is a numeric IP address, that identifies 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. For example, an A record is 123.123.123.123 and an MX record is mx1.domain.com. Whenever you open a website or send an e-mail, 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. In case you have custom records on their end, the web browser request or the email will then 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 service provider and the emails by another.