The word “hosting” does not describe a particular service, but several services which offer different functions to a domain name. Having a website and e-mails, as an example, are two independent services although in the general case they come together, so most of the people consider them as one single service. In fact, every domain name has a couple 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 address is loaded from, while the latter is an alphanumeric string, which shows the server that deals with the e-mails for the domain name. For example, an A record can be 188.8.131.52 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 address has and the traffic/message is first directed to that company. If you have custom records on their end, the Internet browser request or the email will then be forwarded to the correct server. The idea behind employing separate records is that the two services employ different web protocols and you could have your website hosted by one company and the e-mails by another.