The word “hosting” doesn't describe only one service, but a set of services which provide various functions to a domain address. Having a site and emails, for example, are two individual services even though in the general case they come together, so a lot of people consider them as one single service. The truth is, each domain has a number of DNS records called A and MX, which show the server that handles each particular service - the first one is a numeric IP address, which specifies where the website for the domain is loaded from, while the latter is an alphanumeric string, which shows the server that manages the emails for the domain. As an example, an A record would be 18.104.22.168 and an MX record can be mx1.domain.com. Whenever you open a site 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 forwarded to that company. If you have custom records on their end, the Internet browser request or the e-mail will then be directed to the correct server. The concept behind working with separate records is that the two services use different web protocols and you can have your site hosted by one company and the emails by another.