The term “hosting” doesn't describe a single service, but several services which provide different functions to a domain name. Having a website and emails, as an example, are two separate services even though in the general case they come together, so many people consider them as one single service. In reality, each domain has a number of DNS records called A and MX, which show the server that handles each particular service - the former is a numeric IP address, that identifies where the site for the domain name is loaded from, while the latter is an alphanumeric string, which shows the server that deals with the e-mails for the domain name. For instance, an A record is 123.123.123.123 and an MX record can be mx1.domain.com. Each time you open a site or send an email, the global DNS servers are contacted to check the name servers that a domain name 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 be sent to the correct server. The reasoning behind working with separate records is that the two services use different web protocols and you could have your website hosted by one provider and the emails by another.