The term “hosting” doesn't describe a single service, but a variety of services that offer a variety of functions to a domain address. Having a website and emails, as an illustration, are two individual services though in the general case they come together, so a lot of people see them as one single service. Actually, each and every domain has a number of DNS records called A and MX, which show the server that handles each specific service - the former is a numeric IP address, which identifies 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 emails for the domain. As an example, an A record is 220.127.116.11 and an MX record would be mx1.domain.com. Every time you open a website or send an e-mail, the global DNS servers are contacted to check the name servers that a domain name has and the traffic/message is first directed to that company. When you have custom records on their end, the browser request or the email will then be directed to the correct server. The reasoning behind employing separate records is that the two services employ different web protocols and you could have your website hosted by one service provider and the e-mails by another.