The word “hosting” doesn't describe just one service, but a number of services that provide a variety of functions to a domain name. Having a site and e-mails, as an example, are two independent services even though in the general case they come together, so most people see them as one single service. In reality, each domain has a several DNS records called A and MX, which show the server that deals with each specific service - the first one is a numeric IP address, which specifies where the site for the domain is loaded from, while the second one is an alphanumeric string, which shows the server that handles the e-mails for the domain name. As an illustration, an A record can be 123.123.123.123 and an MX record is mx1.domain.com. Every time you open a website or send an email, the global DNS servers are contacted to check the name servers that a Internet domain has and the traffic/message is first forwarded to that company. If you have custom records on their end, the browser request or the email will then be directed to the correct server. The reasoning behind using separate records is that the two services work with different web protocols and you may have your site hosted by one company and the emails by another.