The term “hosting” doesn't describe a particular service, but several services that provide a variety of functions to a domain name. Having a website and e-mails, as an illustration, are two individual services despite the fact that in the general case they come together, so many people consider them as one single service. The truth is, each domain has a several DNS records called A and MX, which show the server that handles each specific service - the former is a numeric IP address, which specifies where the website for the domain name is loaded from, while the latter is an alphanumeric string, which shows the server that handles the emails for the domain name. As an example, an A record can be 18.104.22.168 and an MX record would be mx1.domain.com. Each time 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 forwarded to that company. In case you have custom records on their end, the web browser request or the e-mail will be forwarded to the correct server. The reasoning behind employing separate records is that the two services employ different web protocols and you can have your site hosted by one provider and the e-mail messages by another.