The word “hosting” doesn't describe a particular service, but a set of services that offer numerous functions to a domain address. Having a site and e-mails, as an example, are two individual services though in the general case they come together, so a lot of people see them as one single service. Actually, every domain has a several DNS records called A and MX, which show the server that deals with each particular service - the first one is a numeric IP address, which defines where the website for the domain is loaded from, while the latter is an alphanumeric string, which shows the server that handles the emails for the domain address. As an example, an A record would be 188.8.131.52 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 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 concept behind employing separate records is that the two services employ different web protocols and you could have your site hosted by one service provider and the e-mail messages by another.