The definition of “hosting” doesn't describe a single service, but a set of services that offer a variety of functions to a domain name. Having a website and emails, as an illustration, are two separate services despite the fact that in the general case they come together, so most people consider them as one single service. The truth is, every domain name has a several DNS records called A and MX, which show the server that deals with each specific service - the first one is a numeric IP address, that defines where the website 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 address. For example, an A record can be 188.8.131.52 and an MX record is mx1.domain.com. Every time you open a site 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 browser request or the e-mail will then be sent to the correct server. The reasoning behind using separate records is that the two services employ different web protocols and you may have your site hosted by one service provider and the e-mails by another.