The term “hosting” does not describe a single service, but a number of services that provide various functions to a domain address. Having a site and emails, 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. In fact, every domain has a number of DNS records called A and MX, which show the server that handles each particular service - the former is a numeric IP address, that identifies where the website for the domain address is loaded from, while the second one is an alphanumeric string, which shows the server that manages the e-mails for the domain address. For example, an A record would be 126.96.36.199 and an MX record can be mx1.domain.com. Whenever 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 forwarded to that company. When you have custom records on their end, the Internet browser request or the e-mail will be sent to the correct server. The reasoning 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 emails by another.