The term “hosting” doesn't describe a particular service, but a variety of services that offer a variety of functions to a domain address. Having a site and emails, as an illustration, are two independent services though in the general case they come together, so a lot of people consider them as one single service. In reality, every domain has a couple of DNS records called A and MX, which show the server that manages each particular service - the former is a numeric IP address, which specifies where the website for the domain is loaded from, while the latter is an alphanumeric string, which shows the server that deals with the emails for the domain. For example, an A record is 220.127.116.11 and an MX record is mx1.domain.com. Whenever you open a website or send an e-mail, the global DNS servers are contacted to check the name servers that a domain name has and the traffic/message is first directed to that company. If you have custom records on their end, the browser request or the e-mail will be forwarded to the correct server. The reasoning behind using separate records is that the two services use different web protocols and you may have your website hosted by one service provider and the e-mails by another.