The term “hosting” does not describe one service, but a variety of services which provide various functions to a domain. Having a site and emails, as an example, are two separate services although in the general case they come together, so most people think of them as one single service. In reality, each domain has a several DNS records called A and MX, which show the server that manages each particular service - the first one is a numeric IP address, that defines where the site for the domain is loaded from, while the latter is an alphanumeric string, which shows the server that deals with the e-mails for the domain. As an illustration, an A record can be 126.96.36.199 and an MX record would 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 domain name has and the traffic/message is first directed to that company. When you have custom records on their end, the web browser request or the email will then be sent to the correct server. The reasoning behind employing separate records is that the two services work with different web protocols and you can have your website hosted by one provider and the e-mails by another.