The term “hosting” does not describe just one service, but a number of services which offer numerous functions to a domain address. Having a website and e-mails, as an example, are two separate services despite the fact that in the general case they come together, so many people consider them as one single service. In fact, every domain has a couple of DNS records called A and MX, which show the server that manages each particular service - the former is a numeric IP address, that defines where the site for the domain name is loaded from, while the latter is an alphanumeric string, which shows the server that deals with the emails for the domain name. For example, an A record would be 220.127.116.11 and an MX record is 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. In case you have custom records on their end, the browser request or the e-mail will be forwarded to the correct server. The concept behind using separate records is that the two services use different web protocols and you may have your site hosted by one company and the e-mail messages by another.