The term “hosting” doesn't describe one service, but a set of services which provide a variety of functions to a domain address. Having a website and emails, for instance, are two independent services despite the fact that in the general case they come together, so a lot of people consider them as one single service. In fact, every single domain has a number of DNS records called A and MX, which show the server that manages each particular service - the former is a numeric IP address, that defines 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 name. As an illustration, an A record is 22.214.171.124 and an MX record can be mx1.domain.com. Each 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 directed to that company. When you have custom records on their end, the browser request or the email will be forwarded to the correct server. The concept behind using 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.