The term “hosting” doesn't describe a particular service, but a set of services that offer a variety of functions to a domain name. Having a website and e-mails, as an illustration, are two individual services although in the general case they come together, so most people consider them as one single service. In reality, every single domain has a number of DNS records called A and MX, which show the server that manages each specific service - the first one is a numeric IP address, that identifies where the site for the domain is loaded from, while the second one is an alphanumeric string, which shows the server that handles the emails for the domain address. For example, an A record is 126.96.36.199 and an MX record can be 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 forwarded to that company. When you have custom records on their end, the browser request or the email will then be sent to the correct server. The concept behind using separate records is that the two services use different web protocols and you could have your website hosted by one service provider and the emails by another.