The definition of “hosting” doesn't describe just one service, but a set of services which provide numerous functions to a domain. Having a website and e-mails, for instance, are two individual services although in the general case they come together, so most of the people see them as one single service. The truth is, each and every domain name has a couple of DNS records called A and MX, which show the server that handles each particular service - the former is a numeric IP address, that identifies 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 emails for the domain. For example, an A record can be 126.96.36.199 and an MX record would be 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 domain address has and the traffic/message is first forwarded to that company. If you have custom records on their end, the web browser request or the email will be sent to the correct server. The concept behind working with separate records is that the two services use different web protocols and you may have your site hosted by one provider and the e-mail messages by another.