The definition of “hosting” does not describe a single service, but several services which offer numerous functions to a domain. Having a website and emails, for example, are two independent services despite the fact that in the general case they come together, so most people see 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 particular service - the first one is a numeric IP address, which defines where the website for the domain address is loaded from, while the second one is an alphanumeric string, which shows the server that handles the emails for the domain address. As an illustration, an A record would be 126.96.36.199 and an MX record would be 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 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 sent to the correct server. The concept behind employing separate records is that the two services work with different web protocols and you could have your site hosted by one service provider and the e-mails by another.