The term “hosting” does not describe just one service, but a number of services that offer different functions to a domain. Having a website and e-mails, 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 fact, each and every domain name has a several DNS records called A and MX, which show the server that deals with each particular service - the first one is a numeric IP address, which specifies where the website for the domain is loaded from, while the latter is an alphanumeric string, which shows the server that deals with the emails for the domain. As an illustration, 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 e-mail, 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. If you have custom records on their end, the browser request or the e-mail will be forwarded to the correct server. The concept behind using separate records is that the two services employ different web protocols and you can have your website hosted by one service provider and the emails by another.