The term “hosting” does not describe one service, but a variety of services that offer various functions to a domain name. Having a website and emails, as an example, are two individual services although in the general case they come together, so many people see them as one single service. In reality, every domain name has a number 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 identifies where the site for the domain address is loaded from, while the latter is an alphanumeric string, which shows the server that deals with the emails for the domain. As an example, an A record is 220.127.116.11 and an MX record would be 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 web browser request or the email will be forwarded to the correct server. The concept behind using separate records is that the two services employ different web protocols and you can have your site hosted by one company and the e-mails by another.