The term “hosting” doesn't describe a particular service, but a variety of services which offer various functions to a domain address. Having a site and e-mails, as an illustration, are two separate 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 several DNS records called A and MX, which show the server that deals with each particular service - the first one is a numeric IP address, that specifies where the site for the domain name is loaded from, while the second one is an alphanumeric string, which shows the server that deals with the emails for the domain. For example, an A record would be 123.123.123.123 and an MX record can be mx1.domain.com. Every time you open a site or send an e-mail, 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 Internet browser request or the email will be forwarded 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 site hosted by one provider and the emails by another.