The definition of “hosting” does not describe only one service, but a set of services that offer different functions to a domain. Having a website and e-mails, for instance, are two independent 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 domain name has a couple of DNS records called A and MX, which show the server that handles each particular service - the first one is a numeric IP address, that identifies where the website for the domain name is loaded from, while the second one is an alphanumeric string, which shows the server that manages the e-mails for the domain. For instance, an A record can be 18.104.22.168 and an MX record can be mx1.domain.com. Every time you open a site or send an email, 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 email will be directed to the correct server. The idea behind using separate records is that the two services work with different web protocols and you may have your site hosted by one company and the e-mails by another.