The term “hosting” doesn't describe one service, but a number of services which offer different functions to a domain. Having a site and emails, as an illustration, are two separate services though in the general case they come together, so most people see them as one single service. The truth is, every domain name has a number of DNS records called A and MX, which show the server that deals with each particular service - the first one is a numeric IP address, which defines where the site for the domain address is loaded from, while the latter is an alphanumeric string, which shows the server that deals with the emails for the domain. For instance, an A record is 188.8.131.52 and an MX record would be mx1.domain.com. Each 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 forwarded to that company. When you have custom records on their end, the Internet browser request or the e-mail will then be sent to the correct server. The idea behind employing separate records is that the two services employ different web protocols and you may have your site hosted by one company and the e-mail messages by another.