The term “hosting” does not describe only one service, but several services that offer numerous functions to a domain. Having a site and e-mails, for example, are two separate services though in the general case they come together, so many people see them as one single service. Actually, each domain has a several DNS records called A and MX, which show the server that deals with each specific service - the former is a numeric IP address, that defines where the website for the domain address is loaded from, while the second one is an alphanumeric string, which shows the server that handles the e-mails for the domain name. As an illustration, an A record can be 220.127.116.11 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 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 email will then be forwarded to the correct server. The idea behind using separate records is that the two services work with different web protocols and you could have your site hosted by one company and the e-mails by another.