The term “hosting” doesn't describe only one service, but several services that provide different functions to a domain name. Having a website and emails, as an example, are two independent services even though in the general case they come together, so many people consider them as one single service. The truth is, every domain has a number of DNS records called A and MX, which show the server that deals with each specific service - the former is a numeric IP address, which defines where the site for the domain is loaded from, while the latter is an alphanumeric string, which shows the server that manages the e-mails for the domain. For example, an A record can be 18.104.22.168 and an MX record can be mx1.domain.com. Every time you open a website or send an email, the global DNS servers are contacted to check the name servers that a domain 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 directed to the correct server. The concept behind using separate records is that the two services work with different web protocols and you can have your website hosted by one provider and the emails by another.