The word “hosting” doesn't describe just one service, but a variety of services which provide a variety of functions to a domain name. Having a website and e-mails, 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. Actually, each domain name has a couple of DNS records called A and MX, which show the server that manages each specific service - the former is a numeric IP address, that defines where the site for the domain is loaded from, while the latter is an alphanumeric string, which shows the server that manages the emails for the domain address. As an illustration, an A record is 123.123.123.123 and an MX record is 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 then be sent to the correct server. The concept 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 emails by another.