The definition of “hosting” doesn't describe a particular service, but several services which provide numerous functions to a domain address. Having a site and e-mails, as an illustration, are two individual services although in the general case they come together, so most people see them as one single service. In fact, every domain name has a number of DNS records called A and MX, which show the server that manages each particular service - the first one is a numeric IP address, that specifies where the website for the domain name is loaded from, while the latter is an alphanumeric string, which shows the server that manages the emails for the domain. As an illustration, an A record can be 188.8.131.52 and an MX record is mx1.domain.com. Whenever you open a website or send an e-mail, the global DNS servers are contacted to check the name servers that a domain address has and the traffic/message is first directed to that company. In case you have custom records on their end, the web browser request or the email will be sent to the correct server. The concept behind using separate records is that the two services use different web protocols and you could have your site hosted by one provider and the e-mail messages by another.