The term “hosting” does not describe one service, but a set of services which offer a variety of functions to a domain. Having a website and e-mails, for example, are two separate services despite the fact that in the general case they come together, so many people consider them as one single service. Actually, every single domain name has a number of DNS records called A and MX, which show the server that manages each specific service - the former is a numeric IP address, which specifies where the site 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 can be 123.123.123.123 and an MX record is mx1.domain.com. Each time you open a website 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. When you have custom records on their end, the browser request or the email will be forwarded to the correct server. The idea behind using separate records is that the two services employ different web protocols and you may have your site hosted by one service provider and the e-mail messages by another.