The definition of “hosting” does not describe a particular service, but several services that provide various functions to a domain name. Having a site and emails, for example, are two independent services although in the general case they come together, so most people consider them as one single service. In fact, each and every domain name has a couple of DNS records called A and MX, which show the server that handles each specific service - the first one is a numeric IP address, that specifies where the site for the domain is loaded from, while the second one is an alphanumeric string, which shows the server that handles the e-mails for the domain. As an illustration, an A record would be 123.123.123.123 and an MX record is 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 browser request or the e-mail will be directed to the correct server. The idea behind working with separate records is that the two services use different web protocols and you may have your site hosted by one company and the e-mails by another.