The word “hosting” doesn't describe just one service, but a variety of services that offer numerous 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 think of them as one single service. The truth is, each and every domain name has a number of DNS records called A and MX, which show the server that manages each particular service - the first one is a numeric IP address, that defines where the website for the domain is loaded from, while the second one is an alphanumeric string, which shows the server that handles the emails for the domain. For example, an A record would be 188.8.131.52 and an MX record is mx1.domain.com. Whenever you open a website or send an email, 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 Internet browser request or the email will then be sent to the correct server. The idea behind employing separate records is that the two services employ different web protocols and you can have your website hosted by one company and the emails by another.