The definition of “hosting” doesn't describe a single service, but a variety of services which provide different functions to a domain. Having a site and emails, for example, are two separate services although in the general case they come together, so most of the people think of them as one single service. In reality, each and every domain has a number of DNS records called A and MX, which show the server that handles each particular service - the first one is a numeric IP address, that specifies where the website for the domain address is loaded from, while the latter is an alphanumeric string, which shows the server that handles the e-mails for the domain. For example, an A record is 126.96.36.199 and an MX record is mx1.domain.com. Every time you open a website or send an email, the global DNS servers are contacted to check the name servers that a domain address 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 then be forwarded to the correct server. The idea behind employing separate records is that the two services use different web protocols and you can have your site hosted by one company and the e-mails by another.