The term “hosting” does not describe just one service, but a set of services that provide numerous functions to a domain name. Having a website and e-mails, for instance, are two independent services although in the general case they come together, so many people see them as one single service. Actually, each and every domain has a couple of DNS records called A and MX, which show the server that deals with each specific service - the first one is a numeric IP address, that defines where the website for the domain address is loaded from, while the second one is an alphanumeric string, which shows the server that manages the emails for the domain address. As an illustration, an A record can be 220.127.116.11 and an MX record would be mx1.domain.com. Every 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 forwarded to that company. In case you have custom records on their end, the web browser request or the email will be sent to the correct server. The idea behind using separate records is that the two services use different web protocols and you could have your website hosted by one provider and the emails by another.