The term “hosting” does not describe just one service, but a number of services that offer various functions to a domain. Having a site and e-mails, as an example, are two independent services despite the fact that in the general case they come together, so most people think of them as one single service. In reality, each domain name has a several DNS records called A and MX, which show the server that deals with each particular service - the first one is a numeric IP address, that defines where the website for the domain address is loaded from, while the latter is an alphanumeric string, which shows the server that deals with the e-mails for the domain. As an example, an A record can be 123.123.123.123 and an MX record can be mx1.domain.com. Whenever 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 directed to that company. If you have custom records on their end, the web browser request or the email will then be forwarded to the correct server. The concept behind using separate records is that the two services work with different web protocols and you can have your website hosted by one company and the e-mail messages by another.