The word “hosting” does not describe a single service, but several services which provide different functions to a domain. Having a website and e-mails, for instance, are two independent services though in the general case they come together, so most of the people consider them as one single service. The truth is, every single domain has a several DNS records called A and MX, which show the server that handles each specific service - the former is a numeric IP address, which defines where the site for the domain is loaded from, while the latter is an alphanumeric string, which shows the server that deals with the emails for the domain address. For instance, an A record is 220.127.116.11 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 directed to the correct server. The reasoning behind using separate records is that the two services employ different web protocols and you may have your website hosted by one service provider and the emails by another.