The word “hosting” doesn't describe only one service, but a number of services that provide numerous functions to a domain. Having a site and emails, for example, are two individual services despite the fact that in the general case they come together, so many people see 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 first one is a numeric IP address, that defines where the site for the domain is loaded from, while the latter is an alphanumeric string, which shows the server that deals with the emails for the domain. For example, an A record would be 123.123.123.123 and an MX record is mx1.domain.com. Whenever you open a site or send an e-mail, the global DNS servers are contacted to check the name servers that a domain name has and the traffic/message is first forwarded to that company. In case you have custom records on their end, the web browser request or the email will then be sent to the correct server. The idea behind using separate records is that the two services work with different web protocols and you can have your site hosted by one company and the e-mails by another.