The word “hosting” does not describe only one service, but a set of services that offer various functions to a domain. Having a site and emails, as an example, are two individual services though in the general case they come together, so most of the people see them as one single service. Actually, each and every domain name has a several DNS records called A and MX, which show the server that manages each particular service - the first one is a numeric IP address, that identifies where the website for the domain name is loaded from, while the second one is an alphanumeric string, which shows the server that deals with the e-mails for the domain. As an example, an A record can be 123.123.123.123 and an MX record would be 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 directed to that company. In case you have custom records on their end, the Internet browser request or the email will be forwarded to the correct server. The idea behind employing separate records is that the two services use different web protocols and you may have your site hosted by one service provider and the e-mails by another.