The word “hosting” doesn't describe a single service, but several services which offer different functions to a domain address. Having a website and emails, for example, are two separate services even though in the general case they come together, so most people think of them as one single service. In reality, every domain has a couple of DNS records called A and MX, which show the server that manages each specific service - the first one is a numeric IP address, that specifies where the site for the domain is loaded from, while the latter is an alphanumeric string, which shows the server that manages the e-mails for the domain name. For instance, an A record can be 126.96.36.199 and an MX record is mx1.domain.com. Whenever 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. When you have custom records on their end, the Internet browser request or the email will be directed to the correct server. The idea behind employing separate records is that the two services employ different web protocols and you could have your website hosted by one provider and the emails by another.