The term “hosting” doesn't describe just one service, but a set of services that offer a variety of functions to a domain. Having a website and emails, for instance, are two independent services though in the general case they come together, so most people consider them as one single service. In fact, every single domain name has a couple of DNS records called A and MX, which show the server that handles each particular service - the former is a numeric IP address, which specifies where the site for the domain is loaded from, while the latter is an alphanumeric string, which shows the server that handles the emails for the domain. For example, an A record would be 220.127.116.11 and an MX record would 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 has and the traffic/message is first forwarded to that company. When you have custom records on their end, the Internet browser request or the e-mail will then be forwarded to the correct server. The concept behind employing separate records is that the two services use different web protocols and you may have your website hosted by one provider and the e-mail messages by another.