The definition of “hosting” does not describe a single service, but several services which offer numerous functions to a domain. Having a website and e-mails, as an example, are two individual services even though in the general case they come together, so most of the people think of them as one single service. Actually, every domain has a couple of DNS records called A and MX, which show the server that manages each specific service - the former is a numeric IP address, that defines where the website for the domain is loaded from, while the latter is an alphanumeric string, which shows the server that handles the e-mails for the domain name. As an example, an A record is 22.214.171.124 and an MX record can be mx1.domain.com. Each time you open a site or send an e-mail, the global DNS servers are contacted to check the name servers that a domain name has and the traffic/message is first directed to that company. When you have custom records on their end, the web browser request or the email will be directed to the correct server. The idea behind employing separate records is that the two services work with different web protocols and you can have your website hosted by one service provider and the e-mail messages by another.