The term “hosting” does not describe just one service, but a set of services which provide numerous functions to a domain. Having a website and e-mails, for instance, are two separate 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 reality, each domain name has a couple of DNS records called A and MX, which show the server that deals with each specific service - the former is a numeric IP address, which defines where the site for the domain address is loaded from, while the second one is an alphanumeric string, which shows the server that deals with the emails for the domain address. For example, an A record would be 220.127.116.11 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 address 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 email will be sent to the correct server. The idea 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-mail messages by another.