The term “hosting” doesn't describe just one service, but a number of services that provide different functions to a domain address. Having a website and emails, as an example, are two independent services even though in the general case they come together, so many people think of them as one single service. Actually, every domain has a several DNS records called A and MX, which show the server that manages each specific 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 deals with the e-mails for the domain. As an illustration, an A record is 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 address has and the traffic/message is first directed to that company. If you have custom records on their end, the browser request or the e-mail 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 could have your website hosted by one service provider and the e-mails by another.