The word “hosting” does not describe only one service, but a number of services which offer numerous functions to a domain. Having a website and emails, as an illustration, are two individual services even though in the general case they come together, so many people consider them as one single service. In fact, every domain has a several DNS records called A and MX, which show the server that deals with each specific service - the former is a numeric IP address, that specifies where the website for the domain is loaded from, while the second one is an alphanumeric string, which shows the server that manages the emails for the domain name. For example, an A record would be 123.123.123.123 and an MX record would 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 Internet domain has and the traffic/message is first directed to that company. When you have custom records on their end, the Internet browser request or the e-mail will then be directed to the correct server. The idea behind using separate records is that the two services use different web protocols and you could have your site hosted by one provider and the emails by another.