The word “hosting” doesn't describe only one service, but a variety of services that offer a variety of functions to a domain. Having a website and e-mails, as an illustration, 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, every domain name has a number of DNS records called A and MX, which show the server that handles each specific service - the former is a numeric IP address, that defines where the site for the domain is loaded from, while the second one is an alphanumeric string, which shows the server that manages the emails for the domain name. As an illustration, an A record is 126.96.36.199 and an MX record can 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 Internet domain has and the traffic/message is first forwarded to that company. In case you have custom records on their end, the Internet browser request or the e-mail will be directed to the correct server. The reasoning behind using separate records is that the two services work with different web protocols and you may have your site hosted by one provider and the e-mail messages by another.