The word “hosting” doesn't describe one service, but a set of services that offer different functions to a domain name. Having a site and emails, for example, are two independent services even though in the general case they come together, so most of the people think of them as one single service. Actually, each and every domain has a number of DNS records called A and MX, which show the server that handles each particular service - the first one is a numeric IP address, which identifies where the site for the domain address is loaded from, while the second one is an alphanumeric string, which shows the server that deals with the e-mails for the domain name. As an example, an A record would be 126.96.36.199 and an MX record is mx1.domain.com. Each time you open a website or send an email, the global DNS servers are contacted to check the name servers that a Internet domain has and the traffic/message is first directed to that company. If you have custom records on their end, the web browser request or the e-mail will be forwarded to the correct server. The reasoning behind working with separate records is that the two services work with different web protocols and you could have your website hosted by one service provider and the e-mail messages by another.