The word “hosting” doesn't describe just one service, but a set of services that offer a variety of functions to a domain name. Having a website and e-mails, as an example, are two individual services despite the fact that in the general case they come together, so most of the people consider them as one single service. In reality, every domain name has a number of DNS records called A and MX, which show the server that deals with each specific 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 e-mails for the domain address. For instance, 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 domain address has and the traffic/message is first directed to that company. If you have custom records on their end, the web browser request or the e-mail will then be sent to the correct server. The reasoning behind employing separate records is that the two services use different web protocols and you can have your website hosted by one provider and the e-mail messages by another.