The definition of “hosting” doesn't describe just one service, but several services that provide different functions to a domain. Having a site and emails, for example, are two separate services despite the fact that in the general case they come together, so a lot of people see them as one single service. In reality, every single domain has a number of DNS records called A and MX, which show the server that manages each specific service - the former is a numeric IP address, that defines where the website for the domain address is loaded from, while the latter is an alphanumeric string, which shows the server that manages the e-mails for the domain address. As an example, an A record can be 220.127.116.11 and an MX record can 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 has and the traffic/message is first forwarded to that company. If you have custom records on their end, the Internet browser request or the email will be directed to the correct server. The idea behind using separate records is that the two services use different web protocols and you can have your site hosted by one service provider and the e-mail messages by another.