The term “hosting” doesn't describe a single service, but several services which provide various functions to a domain name. Having a website and emails, as an example, are two individual services even though in the general case they come together, so many people see them as one single service. The truth is, every single domain has a several DNS records called A and MX, which show the server that deals with each particular service - the first one is a numeric IP address, which defines where the website for the domain address is loaded from, while the latter is an alphanumeric string, which shows the server that manages the e-mails for the domain address. For instance, an A record can be 220.127.116.11 and an MX record can be mx1.domain.com. Each time you open a website or send an email, the global DNS servers are contacted to check the name servers that a domain address has and the traffic/message is first forwarded to that company. If you have custom records on their end, the browser request or the e-mail will be directed to the correct server. The concept behind employing separate records is that the two services work with different web protocols and you may have your site hosted by one provider and the emails by another.