The definition of “hosting” doesn't describe a particular service, but a number of services which provide numerous functions to a domain. Having a site and emails, for instance, are two independent services though in the general case they come together, so many people see them as one single service. Actually, every domain has a several DNS records called A and MX, which show the server that handles each specific service - the first one is a numeric IP address, that identifies where the site for the domain name is loaded from, while the second one is an alphanumeric string, which shows the server that deals with the emails for the domain name. As an illustration, an A record can be 126.96.36.199 and an MX record can be mx1.domain.com. Every time 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 Internet browser request or the e-mail will then be directed to the correct server. The idea behind using separate records is that the two services work with different web protocols and you can have your site hosted by one provider and the e-mails by another.