The word “hosting” does not describe only one service, but several services which offer a variety of functions to a domain name. Having a site and emails, as an example, are two individual services although in the general case they come together, so most of the people see them as one single service. In fact, each and every domain has a couple of DNS records called A and MX, which show the server that handles each specific service - the former is a numeric IP address, which specifies where the website for the domain name is loaded from, while the latter is an alphanumeric string, which shows the server that manages the emails for the domain name. As an illustration, an A record can be 126.96.36.199 and an MX record can be mx1.domain.com. Whenever you open a website or send an email, 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 e-mail will be forwarded to the correct server. The concept behind working with separate records is that the two services work with different web protocols and you could have your website hosted by one service provider and the e-mails by another.