The definition of “hosting” does not describe just one service, but a set of services which provide a variety of functions to a domain address. Having a site and e-mails, as an illustration, are two independent services although in the general case they come together, so a lot of people think of them as one single service. In reality, every single domain name has a couple of DNS records called A and MX, which show the server that deals with each specific service - the former is a numeric IP address, which defines where the website for the domain address is loaded from, while the latter is an alphanumeric string, which shows the server that deals with the e-mails for the domain name. As an illustration, an A record would be 126.96.36.199 and an MX record would be 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 has and the traffic/message is first directed to that company. When 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 using separate records is that the two services employ different web protocols and you may have your site hosted by one company and the emails by another.