The word “hosting” doesn't describe a single service, but a variety of services which offer various functions to a domain name. Having a site and emails, as an example, are two individual services although in the general case they come together, so most of the people see them as one single service. The truth is, every domain name has a number of DNS records called A and MX, which show the server that handles each specific service - the first one is a numeric IP address, which identifies where the site for the domain is loaded from, while the latter is an alphanumeric string, which shows the server that manages the emails for the domain address. As an illustration, an A record would be 126.96.36.199 and an MX record would be 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 domain address 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 then be sent to the correct server. The reasoning behind employing separate records is that the two services use different web protocols and you could have your site hosted by one service provider and the e-mails by another.