The definition of “hosting” does not describe one service, but a number of services that provide a variety of functions to a domain name. Having a website and emails, as an example, are two independent services although in the general case they come together, so most people see them as one single service. In reality, each and every domain 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, that identifies where the site 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. As an illustration, an A record is 126.96.36.199 and an MX record is mx1.domain.com. Every time you open a site or send an e-mail, the global DNS servers are contacted to check the name servers that a domain name 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 e-mail will then be directed to the correct server. The reasoning behind working with separate records is that the two services use different web protocols and you can have your website hosted by one service provider and the e-mails by another.