The term “hosting” does not describe just one service, but a number of services that provide numerous functions to a domain. Having a site and e-mails, for example, are two independent services even though in the general case they come together, so a lot of people think of them as one single service. In reality, every single domain name has a number of DNS records called A and MX, which show the server that handles each particular service - the former is a numeric IP address, which defines where the site for the domain address is loaded from, while the latter is an alphanumeric string, which shows the server that manages the e-mails for the domain address. As an illustration, an A record can be 18.104.22.168 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 Internet 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 e-mail will be forwarded to the correct server. The reasoning behind using separate records is that the two services work with different web protocols and you could have your website hosted by one company and the e-mails by another.