The definition of “hosting” does not describe a particular service, but a number of services that offer numerous functions to a domain name. Having a site and e-mails, for example, are two individual services even though in the general case they come together, so a lot of people consider them as one single service. In reality, each domain name has a several DNS records called A and MX, which show the server that deals with each particular service - the first one is a numeric IP address, which identifies where the website for the domain is loaded from, while the latter is an alphanumeric string, which shows the server that deals with the emails for the domain name. For example, an A record would be 184.108.40.206 and an MX record is mx1.domain.com. Whenever you open a website 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 directed to that company. In case you have custom records on their end, the browser request or the e-mail will be forwarded to the correct server. The idea behind employing separate records is that the two services work with different web protocols and you may have your website hosted by one company and the emails by another.