The definition of “hosting” doesn't describe a particular service, but several services that offer different functions to a domain. Having a website and e-mails, 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. The truth is, every domain name has a couple of DNS records called A and MX, which show the server that manages each particular service - the first one is a numeric IP address, which defines where the website for the domain is loaded from, while the second one is an alphanumeric string, which shows the server that handles the e-mails for the domain. For instance, an A record can be 22.214.171.124 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 Internet domain has and the traffic/message is first forwarded to that company. If you have custom records on their end, the Internet browser request or the e-mail will then be sent to the correct server. The concept behind working with separate records is that the two services use different web protocols and you could have your site hosted by one company and the emails by another.