The term “hosting” does not describe just one service, but a set of services that provide various functions to a domain. Having a site and emails, for example, are two independent services though in the general case they come together, so a lot of people think of them as one single service. Actually, each and every domain has a number of DNS records called A and MX, which show the server that handles each particular service - the former is a numeric IP address, which identifies where the site for the domain address is loaded from, while the second one is an alphanumeric string, which shows the server that handles the e-mails for the domain. As an example, an A record would be 188.8.131.52 and an MX record would be mx1.domain.com. Whenever you open a website or send an email, 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 e-mail will then be directed to the correct server. The idea behind employing separate records is that the two services use different web protocols and you can have your site hosted by one company and the emails by another.