The word “hosting” doesn't describe just one service, but a number of services which offer different functions to a domain. Having a website and e-mails, for instance, are two individual services despite the fact that in the general case they come together, so a lot of people think of them as one single service. In fact, each domain name has a couple 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 website for the domain name is loaded from, while the second one is an alphanumeric string, which shows the server that manages the e-mails for the domain name. As an example, an A record can be 126.96.36.199 and an MX record would be mx1.domain.com. Each time you open a site or send an e-mail, the global DNS servers are contacted to check the name servers that a domain address 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 email will then be directed to the correct server. The concept behind employing separate records is that the two services work with different web protocols and you may have your website hosted by one service provider and the e-mail messages by another.