The term “hosting” doesn't describe only one service, but a variety of services that offer different functions to a domain. Having a site and e-mails, as an illustration, are two individual services despite the fact that in the general case they come together, so a lot of people see them as one single service. Actually, each and every domain has a several DNS records called A and MX, which show the server that manages each particular service - the first one is a numeric IP address, which defines where the website for the domain is loaded from, while the second one is an alphanumeric string, which shows the server that deals with the e-mails for the domain address. As an illustration, an A record can be 123.123.123.123 and an MX record can be mx1.domain.com. Every 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 directed to that company. In case you have custom records on their end, the web browser request or the e-mail will be directed to the correct server. The reasoning behind using separate records is that the two services use different web protocols and you may have your site hosted by one company and the e-mails by another.