The word “hosting” doesn't describe one service, but a set of services which offer numerous functions to a domain. Having a website and e-mails, as an illustration, are two individual services although in the general case they come together, so many people think of 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, which specifies where the site for the domain address is loaded from, while the second one is an alphanumeric string, which shows the server that handles the emails for the domain name. For example, an A record would be 220.127.116.11 and an MX record would be mx1.domain.com. Whenever you open a website 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 forwarded to that company. If you have custom records on their end, the Internet browser request or the e-mail will then be sent to the correct server. The reasoning behind working with separate records is that the two services work with different web protocols and you could have your website hosted by one company and the emails by another.