The word “hosting” does not describe a particular service, but several services which provide a variety of functions to a domain name. Having a website and emails, for instance, are two individual services despite the fact that in the general case they come together, so many people see them as one single service. The truth is, every single domain has a couple of DNS records called A and MX, which show the server that deals with each particular service - the former is a numeric IP address, which identifies where the site for the domain is loaded from, while the second one is an alphanumeric string, which shows the server that deals with the e-mails for the domain address. As an illustration, an A record can be 188.8.131.52 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 forwarded to that company. When you have custom records on their end, the web browser request or the e-mail will be directed to the correct server. The reasoning behind using separate records is that the two services work with different web protocols and you could have your site hosted by one provider and the emails by another.