The word “hosting” does not describe just one service, but a variety of services which offer a variety of functions to a domain. Having a site and emails, as an example, are two separate services even though in the general case they come together, so most of the people see them as one single service. Actually, every single domain name has a several DNS records called A and MX, which show the server that deals with each particular service - the first one is a numeric IP address, that defines where the site for the domain address is loaded from, while the second one is an alphanumeric string, which shows the server that handles the e-mails for the domain name. As an illustration, an A record is 126.96.36.199 and an MX record can be 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 name has and the traffic/message is first forwarded to that company. If you have custom records on their end, the Internet browser request or the email will be sent to the correct server. The idea behind employing separate records is that the two services work with different web protocols and you may have your website hosted by one provider and the e-mails by another.