The word “hosting” doesn't describe a single service, but several services which offer a variety of functions to a domain address. Having a site and e-mails, for example, are two independent services though in the general case they come together, so most people think of them as one single service. The truth is, each and every domain has a several DNS records called A and MX, which show the server that deals with each specific service - the former is a numeric IP address, that defines where the website for the domain is loaded from, while the second one is an alphanumeric string, which shows the server that deals with the emails for the domain. As an illustration, an A record is 123.123.123.123 and an MX record is 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 forwarded to that company. If you have custom records on their end, the web 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 use different web protocols and you can have your site hosted by one provider and the e-mails by another.