The definition of “hosting” doesn't describe a particular service, but a variety of services which offer various functions to a domain name. Having a site and emails, for example, are two separate services even though in the general case they come together, so a lot of people consider them as one single service. In fact, every single domain has a couple of DNS records called A and MX, which show the server that handles each specific service - the former is a numeric IP address, that identifies where the site for the domain address is loaded from, while the latter is an alphanumeric string, which shows the server that manages the e-mails for the domain. As an example, an A record can be 123.123.123.123 and an MX record would be mx1.domain.com. Every time you open a site or send an email, the global DNS servers are contacted to check the name servers that a domain has and the traffic/message is first directed to that company. When you have custom records on their end, the Internet browser request or the e-mail will be directed to the correct server. The concept behind working with separate records is that the two services employ different web protocols and you can have your site hosted by one provider and the emails by another.