The word “hosting” doesn't describe just one service, but a set of services that provide different functions to a domain address. Having a website and emails, for instance, are two separate services despite the fact that in the general case they come together, so a lot of people think of them as one single service. The truth is, every single domain has a number 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 identifies 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 e-mails for the domain. For example, an A record would be 126.96.36.199 and an MX record would be mx1.domain.com. Each time you open a website 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 directed 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 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.