The word “hosting” does not describe a particular service, but a number of services that provide various functions to a domain address. Having a site and e-mails, for example, are two separate services though in the general case they come together, so most of the people see them as one single service. Actually, every single domain name has a couple of DNS records called A and MX, which show the server that handles each particular service - the first one is a numeric IP address, that identifies where the website for the domain is loaded from, while the latter is an alphanumeric string, which shows the server that handles the emails for the domain. As an illustration, an A record is 184.108.40.206 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 has and the traffic/message is first directed to that company. If you have custom records on their end, the web browser request or the email will then be forwarded to the correct server. The reasoning behind employing separate records is that the two services use different web protocols and you may have your site hosted by one service provider and the e-mails by another.