The term “hosting” doesn't describe a particular service, but several services which provide a variety of functions to a domain address. Having a site and emails, for instance, are two individual services though in the general case they come together, so most of the people consider them as one single service. The truth is, each domain has a number of DNS records called A and MX, which show the server that manages each specific service - the former is a numeric IP address, that identifies 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 e-mails for the domain address. For example, an A record is 126.96.36.199 and an MX record is mx1.domain.com. Whenever you open a site or send an email, the global DNS servers are contacted to check the name servers that a Internet domain has and the traffic/message is first directed to that company. When you have custom records on their end, the browser request or the email will be sent to the correct server. The concept behind working with separate records is that the two services work with different web protocols and you can have your site hosted by one provider and the e-mails by another.