The definition of “hosting” doesn't describe only one service, but a set of services that provide various functions to a domain name. Having a website and e-mails, for example, are two separate services although in the general case they come together, so most of the people think of them as one single service. In fact, every domain name has a several DNS records called A and MX, which show the server that handles each specific service - the former is a numeric IP address, that defines where the website for the domain address is loaded from, while the latter is an alphanumeric string, which shows the server that handles the emails for the domain. As an example, an A record can be 126.96.36.199 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 domain has and the traffic/message is first directed to that company. When you have custom records on their end, the Internet browser request or the email will then be forwarded to the correct server. The reasoning behind using separate records is that the two services use different web protocols and you may have your website hosted by one service provider and the emails by another.