The word “hosting” does not describe only one service, but a number of services which provide various functions to a domain. Having a website and emails, as an illustration, are two separate services even though in the general case they come together, so most people consider them as one single service. In fact, every single domain has a couple of DNS records called A and MX, which show the server that handles each particular service - the first one is a numeric IP address, that specifies where the site for the domain name is loaded from, while the second one is an alphanumeric string, which shows the server that deals with the emails for the domain. As an illustration, an A record is 126.96.36.199 and an MX record can 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 address has and the traffic/message is first forwarded to that company. If you have custom records on their end, the web browser request or the e-mail will then be forwarded to the correct server. The reasoning behind using separate records is that the two services employ different web protocols and you may have your website hosted by one service provider and the e-mail messages by another.