The definition of “hosting” doesn't describe a single service, but a number of services which offer numerous functions to a domain address. Having a website and e-mails, as an illustration, are two independent services even though in the general case they come together, so most people consider them as one single service. The truth is, every single domain has a number of DNS records called A and MX, which show the server that deals with each specific service - the first one is a numeric IP address, which specifies where the site for the domain is loaded from, while the second one is an alphanumeric string, which shows the server that handles the e-mails for the domain. For example, an A record is 188.8.131.52 and an MX record would be 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 domain has and the traffic/message is first forwarded to that company. If you have custom records on their end, the browser request or the email will be directed to the correct server. The reasoning behind using separate records is that the two services work with different web protocols and you could have your site hosted by one provider and the e-mails by another.