The term “hosting” doesn't describe only one service, but a set of services that provide different functions to a domain address. Having a website and e-mails, for example, are two independent services despite the fact that in the general case they come together, so many people see them as one single service. In reality, each domain name has a several DNS records called A and MX, which show the server that manages each specific service - the first one is a numeric IP address, which identifies where the site for the domain address is loaded from, while the latter is an alphanumeric string, which shows the server that deals with the emails for the domain address. As an illustration, an A record is 18.104.22.168 and an MX record is mx1.domain.com. Every time you open a site or send an e-mail, 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 web browser request or the e-mail will then be directed to the correct server. The reasoning behind using separate records is that the two services employ different web protocols and you can have your site hosted by one service provider and the e-mail messages by another.