The definition of “hosting” does not describe a single service, but a number of services which provide a variety of functions to a domain. Having a site and e-mails, as an example, are two separate services though in the general case they come together, so most of the people consider them as one single service. In fact, every domain has a couple of DNS records called A and MX, which show the server that deals with each particular service - the former is a numeric IP address, which specifies 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 e-mails for the domain name. For example, an A record would be 188.8.131.52 and an MX record can 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 Internet domain has and the traffic/message is first directed to that company. In case you have custom records on their end, the Internet browser request or the e-mail will be forwarded to the correct server. The concept behind using separate records is that the two services use different web protocols and you may have your website hosted by one company and the e-mails by another.