The word “hosting” doesn't describe a particular service, but a set of services that provide different functions to a domain name. Having a website and emails, as an illustration, are two separate services although in the general case they come together, so a lot of people consider them as one single service. Actually, every single domain has a number of DNS records called A and MX, which show the server that manages each specific service - the former is a numeric IP address, which specifies where the website for the domain address is loaded from, while the second one is an alphanumeric string, which shows the server that manages the e-mails for the domain address. For example, an A record can be 220.127.116.11 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 directed to that company. If you have custom records on their end, the Internet browser request or the email will then be sent to the correct server. The reasoning behind working with separate records is that the two services use different web protocols and you could have your website hosted by one provider and the emails by another.