The definition of “hosting” does not describe only one service, but a variety of services that offer a variety of functions to a domain. Having a website and emails, as an example, are two separate services though in the general case they come together, so many people think of them as one single service. The truth is, each domain has a number of DNS records called A and MX, which show the server that handles each particular service - the first one is a numeric IP address, that defines where the site for the domain address is loaded from, while the latter is an alphanumeric string, which shows the server that manages the emails for the domain. For instance, an A record is 126.96.36.199 and an MX record would be mx1.domain.com. Every time you open a website 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 forwarded to that company. In case you have custom records on their end, the Internet browser request or the email 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 website hosted by one service provider and the e-mails by another.