The word “hosting” does not describe only one service, but a variety of services that offer numerous functions to a domain name. Having a site and emails, for example, are two individual services though in the general case they come together, so most of the people think of them as one single service. Actually, every single domain has a number of DNS records called A and MX, which show the server that deals with each particular service - the first one is a numeric IP address, that defines where the site for the domain address is loaded from, while the latter is an alphanumeric string, which shows the server that deals with the emails for the domain address. As an illustration, an A record is 123.123.123.123 and an MX record can be mx1.domain.com. Each time you open a website or send an e-mail, the global DNS servers are contacted to check the name servers that a domain name has and the traffic/message is first directed to that company. If you have custom records on their end, the web browser request or the email 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 can have your site hosted by one service provider and the emails by another.