The term “hosting” does not describe only one service, but a set of services which provide different functions to a domain. Having a website and e-mails, for instance, are two independent services 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 handles each specific service - the first one is a numeric IP address, that specifies where the site for the domain address 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 184.108.40.206 and an MX record is mx1.domain.com. Every 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 forwarded to that company. In case you have custom records on their end, the browser request or the email will be directed to the correct server. The concept behind using separate records is that the two services use different web protocols and you may have your site hosted by one service provider and the emails by another.