The term “hosting” does not describe just one service, but a set of services that provide a variety of functions to a domain name. Having a website and emails, as an example, are two separate services despite the fact that in the general case they come together, so most people see them as one single service. In reality, every single domain name has a couple of DNS records called A and MX, which show the server that handles each specific service - the former is a numeric IP address, that defines where the website for the domain is loaded from, while the second one is an alphanumeric string, which shows the server that handles the emails for the domain address. As an example, an A record is 18.104.22.168 and an MX record is mx1.domain.com. Every time you open a website or send an email, the global DNS servers are contacted to check the name servers that a Internet domain has and the traffic/message is first directed to that company. In case you have custom records on their end, the web browser request or the email will be forwarded to the correct server. The reasoning behind using separate records is that the two services work with different web protocols and you may have your website hosted by one company and the emails by another.