The term “hosting” does not describe a single service, but a number of services which offer a variety of functions to a domain. Having a website and emails, for example, are two individual services although in the general case they come together, so most of the people think of them as one single service. In reality, each domain name has a couple of DNS records called A and MX, which show the server that manages each particular service - the first one is a numeric IP address, which specifies where the site for the domain is loaded from, while the latter is an alphanumeric string, which shows the server that handles the emails for the domain address. For example, an A record would be 123.123.123.123 and an MX record is mx1.domain.com. Whenever you open a website 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 email will then be directed to the correct server. The idea behind employing separate records is that the two services work with different web protocols and you may have your site hosted by one provider and the e-mail messages by another.