The definition of “hosting” does not describe a single service, but several services which offer different functions to a domain. Having a website and emails, for instance, are two individual services though in the general case they come together, so a lot of people see 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 deals with each particular service - the former is a numeric IP address, that defines where the website for the domain address is loaded from, while the second one is an alphanumeric string, which shows the server that deals with the emails for the domain. As an illustration, an A record can be 126.96.36.199 and an MX record is mx1.domain.com. Each time 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. In case you have custom records on their end, the web browser request or the email will then be forwarded to the correct server. The reasoning behind using separate records is that the two services work with different web protocols and you can have your site hosted by one provider and the e-mail messages by another.